Well I thought for Athens the different classes all rowed... maybe on different oar banks but- lots of differences in wages could make a large impact though over a 6 month period. I can't imagine the team will make many different ship types for each culture as that could take many unit slots. Not that naval combat and navies weren't important- they were decisive in many campaigns but the auto battles in MTW2 removes much of the impact of special units per faction. So I'd guess they will settle from some representational average. Probably with upkeep tilted a bit more than construction costs as even slaves had some cost and not everyone on the ship were slaves.