Honestly, I rarely have that much money. The way I set up my economy precludes generating large amounts of cash, instead it gets funneled into manufacturing and research (44-44-12). Most of my actual cash turns into technology that I buy from other races. My general goal is for per-turn income to be as close to zero as possible, if I have gobs of cash accumulating then something is broken.
It really depends on which race you are playing.
I recently decided to replay through each of the tech trees to see if I could expand my normal strat and play to the archetype of the race. While playing as the Iridium Corporation, I found that since all of their influence techs are also money techs that it was very easy to simply bankroll buying stuff. I usually rushed planets first, and then ships. However, with the way the starbases now request constructors, I found I did flip through the ship yards to move stuff around and did not use the quick build tab as often as I have previously.
