One can think as everything that comes with investing in tech/buildings as "free" but then and all faction get free staff or everything that comes with tech/building incorporated in the cost of that tech/building.
Since the game generally offer a discount for bundled things, many of things comes cheaper or some one else might claim the ones pay extra for the option to, if and when he wants to unlock something.
The rest is simply "smoke and mirror" from specific users in an attempt to create an imaginary "advantage" of the axis factions have because some things come bundled with tech/buildings.
Yes, all factions do get free stuff and some also pay for others. Like soviet do specifically for the con upgrades.
Things unlocked via teching armt extra costs, they are delayed in their timing. You could look at obers for an example of this. They sued to come from schwere and could buy their lmg, now they come from part 1 of the schwere and part 2 unlocks the LMG. The LMG isn't more expensive now, nor are obers cheaper, all that changed was timing. If something was attached to ukf bofors/AEC tech I'd argue that that is an extra, but the second arty piece in the tank tier is free and gated by time. Grens lmg is free with tech (well I mean... You pay munitions for it but you know what I mean) so is 7th man for cons, though you can pay extra to get it sooner. Rifle nade for grens is free, molotov is not. Faust is very much free, at nade is very much not.
You can't really attach a cost to units held back only by tech because they don't have a fixed value nor a fixed price. Volks stgs don't change in price depending on if you went med or mech. Their power level doesn't fluctuate. You don't think "damn it! I paid an extra 20 fuel to unlock these by going mech! Why don't they work better!" because they are free, when you tech up. However you chose to do that is up to you.
This is why, imo ost is the best faction as a baseline for balance as well as (in theory) the easiest TO balance. They have many steps to teching that provide a multi faced approach to finding the right timing for everything. In the enemd you don't pay more or less for a unit/ ability if it comes with teching. It just comes sooner or later based on where in the teching it's unlocked.