yeah, im still confused as to how you have large amounts of cash and avoid destroying all the AI's crap. End game, im barely pulling in 500 BCs a turn and i usually spend it buying moral buildings for planets i flip from the AI (they either have everything built, or sac their own planets so i have to rebuild everything). I only build the following on my planets (and in this order):
Soil Enhancement
Habitat Improvement
Entertainment Center
Manufacturing Center
AMPP
Banking Center
Economic Exchange
Multimedia Center/Harmony Generator (depends on where i am in game, and how bad im doin on moral)
Stock Exchange
Stadium
Teleporters
Terraforming
and on planets that i use to build stuff: fusion power plants, before manufacturing centers (due to their quick build time).
Oh, and my tax rate starts at 50%, and drops down to 40% once i start making money at 100% spending.