Hi!
The easiets question first: ships' mainenance is 3% of original cost, but maintenance costs for unarmed ships are not charged. If a player fighs only one war at a time, he also doesn't need so many warships that costs of maintenance would be a problem.

How much a ship will cost depends on you and of what you want the ship to be capable off. If you want a fast colonizer to take that big planet before AIs could, you'll probably design one with lots of engines, and pay for it twice the price as for a "normal" colonizer. OTOH for upgrading close econ starbase you may use a tiny or small hull with only the constructor module, as this ship will need neither speed nor range.

For warships the player designs what he can with his current tech, but just builing warships for having them is a waste. Low-tech warships too qickly became obsolete, so it's a good idea to avoid them, and build only fake defenders. Much of the game the player doesn't ask "how much a ship will cost", but "can it make the job done". The logic behind is simple: I rather have one single expensive ship, that can alone wipe the opponent's defending fleet and survive, than a dozen of "normal" price ships, most of them will be destroyed when attacking my opponent. Only in the late game and when not in an overwhelming lead I see the question of ship prices really valid, but in such a situation there's usually not much choices anyway - again the player uses what he has available, and that's the best weapons and defenses in the biggest hull. The only exception to that I see with penultimate weapons - those sometimes give more bang for the bucks, esp. the Positonic Torpedo II.

BR, Iztok