I don't see making outpost ships more expensive as the solution but more of a tweak - really making them cost more simply means that players who have resources will continue to have more resources; Obviously if it's way more cost effective to just take a planet then that's not really balanced either. The more important measure by far is to have a throttling mechanism on the rate of expansion. If you can control a limited number of planets that applies to both planets gained through construction and conquest.