Ok, I said I would have done it, so here it is:
Having the TUV being the sum of the cost of the unit plus the cost of other units that unit consumed is an improvement but has some limits, that make preferable allowing the mapmaker to tell what is the TUV, exactly, namely:
We cannot assume that the upgrade process is necessarily a straight and mandatory production process, in which we intend to reach the final unit since the start. It may be used as a way to convert a unit into something else, not even necessarily something absolutely better or superior, or you may have units that are exactly the same, in term of what they do (thus their actual value), but differ in that are produced in different steps, and the final TUV would differ.
The normal example is WWII Global, in which you have "factory_upgrade" and "factory_major". They are the same (I don't know and I've never even played Global, please anyone correct me if I'm wrong) but factory_upgrade costs 20 and upgrades factory_minor, that costs 12. Thus the TUV would be 32, but it should rather be 30; the same as what a factory_major would cost, if simply placed.
On the other hand, you may have a custom map in which you can decide to produce a factory fast at a higher price or do it in two steps at a lower total price; you would have two factory units that are worth the same, but having different TUV. Rather better both being set to the TUV of the one-step one (especially from the point of view of another player capturing any of them, if they are worth the same).
You may have a map that allows you to produce a carrier or convert a battleship into a carrier_upgrade, while both the carrier and the carrier_upgrade being just the same, thus better having the same TUV (likely, in this case, the "carrier_upgrade" should have a set TUV equal to the "carrier"). Of course, they both might be upgrades, as you might be required to get a hull or something, for the normal "carrier". Maybe the "carrier_upgrade" should even have a lower TUV than the "battleship" it consumes (surely realistic). Or maybe you want to be able to produce "auxiliary_carrier" directly or upgrading a "transport"; probably better them having the same TUV, if these units are different for no other thing but whence they came from.
You may have units that have the upgrade being absolutely worse than the consumed unit.
Aside from madness, this can be used as representing a deliberate scorched heart policy, in which you can "upgrade" "Factory" to "Factory_demolished", if you think it is going to fall into enemy hands. Maybe this process might imply a small "demolition" cost, thus the "Factory_demolished" would end up being valued more than the "Factory". Of course, this would imply that we intend "Factory_demolished" being definitively so, because, if we want to be able to upgrade back to "Factory_something", we would end up in an infinite loop of infinite units needed to infinitely upgrade and downgrade them back an forth (probably we would just do something with triggers, about that, or use user actions).
Probably here the AI should just avoid ever doing it (thus a TUV setting and the AI never upgrading units to other ones that have a lower TUV set).
You can buy resources, not only units. Buying resources using other resources is kind of the same concept as placing a unit consuming another unit.
A TUV setting would allow to cover that case too; otherwise it would need to be accounted too, for completeness.
You may have units that upgrade in multiple steps. This is just a notice in case you intended to look only at the direct upgrades. Either this should be extended to sum up the all the consumed units of the consumed units, or letting the mapmaker making the math and telling the TUV.
An example may be having "keel" to "hull" to "battleship". And maybe during each of those steps we also consume other things, like units "steel" or whatever. And maybe any of those units required some other resources than PUs. And maybe these other resources than PUs were bought by spending PUs. I guess all this can be somehow supported, but probably letting the mapmaker do the math would be feasible the most.
But if we count the consumed units of the consumed units, with potentially no end (as per the above point), you may have units that upgrade units they are the upgrade of, thus being able to keep switching between one and the other, depending on what you need; in this case, if you count all, as per the above point, you will end up having infinite TUV.
For example, you may have a map that automatically gives you "infantry", like with a trigger chance roll that adds infantry units depending on whatever, or maybe with user actions, but you are also able to deliberately upgrade "infantry" to "cavalry" (cavalry consumes infantry when placed) and upgrade "cavalry" back to "infantry" (infantry consumes cavalry when placed), thus managing your armed forces (maybe cavalry is better but has an higher upkeep cost).
I've never actually tried to do something like that, and I guess it is nothing envisioned; this is mostly just a warning, in case. It would be just needed that the TUV stops to be summed up as soon as you get to a unit that you already accounted.
There are a number of other cases in which the TUV is not representative, beside the case of consumed units, and it would probably be not that feasible to go after them all. So it might be argued going after none of them at all, comprising the consumed units one, and just have a setting and let the mapmaker sort any of them out.
I believe the most important case, for the basic maps, is the "Shipyard" tech, that makes the TUV of your ships to sink, just because they are now cheaper, but it may be argued that both the old and the new ships you have should refer to their normal TUV, not the one just lowered by the "Shipyard" tech.
You may have different players buying a same unit at different costs, and a mapmaker may prefer rather using the same TUV for all, or maybe not (a setting would allow to choose).
With this said, I agree that having units in which all the upgrades till the last one is representing a normal production process, like the one for producing a "battleship" in Total World War, is the best referring situation (even tho this is not what WWII Global does, where the upgrade is an alternative less efficient way to get the same thing as a factory_major), thus what you suggest would be surely an improvement (not really an improvement for WWII Global, beside lowering the spread, from a -10 to a +2).
I would really like seeing this getting done, as well, actually, if summing up the consumed units is an easy fix, as if you have a "castle" unit that consumes a "building_castle" unit, it would be really better the TUV being the sum of those, and it is annoying it is not (it kind of defeat almost totally the purpose of the TUV in Stats).
I think the main item that should be assured is that, if this is done, it applies for all consumed units of consumed units as well, as in the case of "keel" to "hull" to "battleship" made above (counting "keel" + "hull" + "battleship"; not just "hull" + "battleship").
Definitely, having the TUV sinking by 2 when you upgrade a hull to a battleship, at cost +10, is not an acceptable behaviour. I'm sure everyone can only agree, here. It really kills the point of seeing the TUV in stats at all, and it is annoying.
Actually, as a related note, I think that the case of "Global" of having different units for the "factory_major" and "factory_upgrade" is just wrong and sort of not the right way to go, especially considering that they look exactly the same and we would be prompted to place both if we click on a territory having a "factory_minor" and allowing for placing either. If they are just the same unit produced in two different ways, they should rather be one and the same unit, really.
If a Global player can confirm that I'm not misunderstanding anything (I've never played global and barely know it), I would rather suggest having a property that instantly turns "factory_upgrade" into "factory_major" right upon placement (thus you would have only a same "factory_major" unit placed in two different ways, as it seems it is the case (please, correct me if I'm wrong)).
Such property would also cover the need for a specified TUV because, say, then a mapmaker could make a "battleship" unit costing 22 PUs that is never actually bought or present in any frontiers, but it is automatically created when you place the 10 PUs "battleship_upgrade", consuming the "hull". I'm not actually sure if this would be the preferable way to go, so to support both current main cases the same way (the Global factory_upgrade and the TWW battleship)? This would allow for managing any upgrades the same way and would remove both the need of having to count all the consumed units of upgrades and having the ability to set a TUV for the unit, plus it would allow to support same units that are produced in different ways, beside only the upgrade versus one-step case of the major factory of Global (say, being able to place a same unit at different costs, depending where you place it, and many other cases).
