Testing with Elite engis:
-placed 3 engis, and later replaced them with SEALs. One was green, the second veteran, the third elite.
I then tested what hitting Y would give me, turns out the "value"/"worth" stays the same: the original cost. Or in the case of vehicles, the value/worth is the build-cost at the current moment, respecting Industrial Plants and any other cost reduction stuff.
I tested to see how refund with Grinder would work, and it always gives the same cash back, regardless of promotion and Industrial Plant reductions.
In the case of vehicles that'd be the full cost, with infantry only half.

-built a super cheap Flak Track (normally 500$ just like Tesla Trooper, but with 4 Industrial Plants it goes to less than 200$) and made it kill 3 Allied Engis costing 500 each, and then a Soviet Engi costing 1.
The Flak Track was only promoted after killing all of them, so 1501$.
This means that cost reductions of Industrial Plants have no influence at all when it comes to veterancy.
And even after the promotion, the refund value was the same as a normal, green Flak Track, and it also ignored any Industrial Plants.

-used a Tesla Trooper to kill an Elite Engineer. He instantly became veteran. Then used him to kill another Elite Engineer. He instantly became Elite.
So it's safe to assume an Elite Engi is worth at least 1501$, but less than 3001$. Two would make at least 3001$.

-used an IFV (cost 600$) to kill two elite engis, and only after that did it become veteran. So it's safe to say two elite engis together are at least 1801$, but less than 3601$ (otherwise the IFV would have become elite).

-placed a neutral SEAL (1000$) and had him kill my elite Tesla Trooper (green 500$) and two spies (together 2000$).
He got promoted to veteran, so an Elite Tesla Trooper must be at least 3001$ - 2000$ = 1001$ (twice his original value) but must be less than 6001$ - 2000$ = 4001$ (else the SEAL would have become elite).

Testing with veteran engis:
-used a Tesla Trooper to kill two veteran engis, he became veteran. So two veteran engis together cost at least 1501, less than 3001.
Four veteran engis make a Tesla Trooper elite, so 4 veteran engis are at least 3001.

-Tesla Trooper kills a veteran engi (??$), a normal engi (500$), and finally the 1$ engi. Only after the last one does he become veteran.
For veteran 1501$ is needed. So a veteran engi must be 1000$.


Based on these tests, I think veteran units are twice as worth as normal/original cost, and elite either twice as worth as veteran, so 4x the normal cost, or just 3x the normal cost.

But this is where things get crazy: mod the Tesla Tank to cost 1000$ (or use a Mirage Tank; it costs 1000$ too).
If you kill two elite engis, it becomes elite. So two elite engis must be at least 3001$. So an elite engi must be at least 1500.5$.
If you kill one elite engi, three normal engis (1500$ in total), then the 1$ engi, it becomes veteran. So an elite engi must be 1500$.

It should be noted that the game supports the option of a country being able to produce units and/or buildings cheaper than the others, despite them being the same for everyone. This has been a feature since RA1, and while still enabled in YR, has simply been unused.
For example: I could give Russia a discount of 10% on all vehicles, and America a discount on Infantry of... I don't know, 99%?
I have no clue how that would affect veterancy.