Reasoning (What will this fix?):
...well I would let the price of the units vary, but the buildings can all have the same price as you can build very little on a planet anyway. I may decide to let some units go up in price rapidly and others more slowly. Say that the price of the next ISD is 125% (maybe even more) while the price of the next X-Wing squad is 110%. That also makes it much cheaper to replace a small fleet as compared to a large fleet. This mean that the 5th ISD will cost 2,44140625 times the normal price (and the formula for the price of an ISD will be [real price]=[start price]*1.25^[ISDs you have]
Current Perceived Imbalance (What is the current scenario of imbalance):
1 Varied and balanced fleets and ground troops, no more building the biggest baddest ships.
2 Small raids while still building up your forces. Really good for the Rebels (as well as making the Rebels feel like the Rebels). It may actually be well worth it to build a few (cheap to replace) Y-Wings only to do a quick strike against an (irreplaceable) ISD and hope some of them get out alive. For the Empire to do a raid, they still have to get a more expensive Acclamator in and risk losing it.
3 as explained by 2, the faction will feel more like they should feel.
4 The strong won't be getting stronger fast, but if you're fleet has just been wiped out, you quickly get a few ships back. You may even be able to get 2 high end ships for the price they would have to pay for a middle class or even a low end ship.
Prevents situations like huge Mon Calamari/Corvette rushes, and forces the fleets to be more versatile.
At least for Galactic Conquest, why not for skirmish too.