@Franton, I agree with OP, "any" ship is what he said. At this time mercenary, commander and salvaged ships are not upgradable, we also should be able to upgrade ships to a different size, at a high cost in a shipyard. Right now ship upgrades are useless, it is easier and cheaper to just build new which means losing all the experience gained by the crews, another option would be the ability to recycle the crew to a new ship of any size thereby retaining the experience points.

Personally I do agree that upgrading standard ships is too expensive, hardly worth it, with few exceptions (upgrading sensor ships, transporters or colony ships to longer range, better sensors or engines is not expensive - it's more about the combat vessels)

I'm a bit more sceptic on the "upgrade _anything_" issue - I mean if you can turn mercs into anything this would probably collide with some of them being over-capacity (I think), which could be OP if upgraded further.