There's a bug, or at least there used to be, with building starbases. You can have 1 construction ship start the starbase and pay the $$ for it (cost might be $250 on marathon but it's cheaper on faster speeds), then cancel that unit's orders. Have another CS construct a starbase on the same turn for its action, then cancel the build starbase order. I've actually stacked 11 CS's and build a starbase in 2 turns in a marathon game before. However, they built 11 starbases instead of just one, and only the original CS was lost. That felt like a cheat so I disbanded all of the starbases but one and disbanded all the CS's as well.