Cost for TECHING is about timing. Cost for units is to represent performance.
In both cases its true, but there is a little of timing on both.
Teching cost affects timings, its the most notable example, as they open the tech tree to new units they limit availability of such units. But the first appearance of a unit is determined by its cost, that is a more subtle timing though. The fuel cost of tanks is a good example of timing too.
With regards to performance we could agree if we both mean of same faction units, also cost limit/ease the player strategy with said faction. Now if we talk of cost of units between opposite factions, we might disagree on some points.
1.
Cheaper is worse fallacy: I hear a lot that "cons are bad because they are cheap", relatively speaking they are cheaper than other squads, true, but it doesnt mean they have to be bad. A counter-example is MG42, its cheap and top tier of MGs, the reason its because OST design encourages the player to use such an efficient unit. On the case of SU cons, its cost limit/enhance the unit spammability in this case.
The opposite is quite true too,
a more expensive unit does not mean it has to overperform. Sure its frustrating to save a lot of time and resources to dump into a unit that cant get a single thing done, but the cost is telling the player to consider more seriously if he/she should invest the resources or to give away that combat. Sometimes you can get a bigger gun but its not the smartest or the best way to win.
To summarize,
i think a unit performance is determined by the faction design, the current metagame and game timings
2.
Cost justifies units toolkit: If a unit has more abilities or more powerful vets, its cost adjusts to include them, i would say that costs includes more aspects than average players normally take account, even the never used abilities.
The possible combinations within the faction and with other allied factions are taken account, that plus the unit intended role end up affecting its costs. This way units can be layed out as cost-effective units/core units/premium units. Keep in mind i am not comparing units between factions yet. A premium unit is one in a faction that shouldn't have such tool. Imagine mobile mortars on UKF, they are a premium since they are doctrinal. I dont remember its costs now but you get the idea.
3.
Cost efficient combat: This might me the hardest to agree of all. If a unit is able to deal cost efficient damage that is an intended role too,
the cost efficient part is the consequence, not the cause. Imagine ostruppen fighting off IS. Of course IS are better overall infantry but if we suppose the case of cover to cover combat,
ostruppen might hold the line same as grens, but with less manpower investment. Mortars are cost efficient vs garrisons, tank destroyers are cost efficient versus same tier tanks. All intended roles, based on the premise there is a risk involved that the enemy could change plans or never field the intended units. There is a complex relationship between proactive gameplay and reactive gameplay, one gives birth to cost efficient units and the other to counterable units. Both points of view aim at the same mechanic. With the ostruppen example, they dont counter IS in any way but they do offer map control in early game, which is more a proactive tactic than a reactive one.
All of this determines a unit risk/reward factor and it then sets its ideal unit cost.
With the unit cost fully elaborated, there is one last point about timings. All factions timings are loosely tied together, because all factions use the same resources for simplicity and all of them come from time passing by. Of course one could capture fuel/muni points to earn some/more but that only promotes map control and strategic thinking. Munition cost limit abilities uses and fuel costs are the second, more exclusive resource income. All resources are infinite, as long as the game continues and VPs do not end.
In the end the best cost efficient units or the safest generalists are always fielded, one depletes the enemy resource pit and the other solidifies a VP. Units cost and build time limit how often they can be replaced and veterancy focuses on units preservation instead of throwing them off like a C&C game.
But it all started and ends as timings stacked up as resources.