I haven't played Sins of a Solar Empire yet, however I can answer the question about units cost and balance.
When an RTS game comes out, it is always pretty well-balanced in skirmish matches against the AI or during the campaign. RTSes are also pretty balanced in low-average levels of online multiplayer experience. However, when it reaches the higher/elite levels of multiplayer gameplay, the super experienced players always and always find a supreme strategy, according to which 50%-70%-90% of the units are completely useless, cost-inneffective, or taking too much time to build. Their strategies are usually early rushes of 2-3 units combined with ultimate micromanaged techniques and the more players gather to the higher/elite levels (and they will reach this level of veterancy, sooner or later) the more "unbalanced" and "bad" the game seems. Because of that, the companies always have to create patches for their RTS games in order to change the building cost, time, hit points, damage etc. of the units and to keep the game alive.
So, if you ask me, it is not the mistake of Sins of a Solar Empire for this unbalance, but the fact we always struggle for something more, we always push the game to its limit and, sooner or later, the game will prove it has a balance issue. Then, we claim the game is "broken to thit" and thigs like that. A game will always be "broken to thit" if you push it to its upper limits, no exception here! If a player believes he/she is such a great player and he/she have found the unbeatable strategy, just give us a brake and either stop playing with such a competitive way or stop playing the game at all!
I know my opinion may be biased or something, but every experienced RTS player will find a deep truth in my words...