that was going to be my next question: is there a point of diminishing returns on a starbase? Can the upkeep outweigh the economic bonuses received?
End of quote
There are points of diminishing returns on certain effects starbases influence, but not on the starbases specifically.
Production at a colony encounters a point of diminishing returns when the bonus production reaches 10×PQ, and a second when normal production reaches 30×PQ.
Tax income at a colony reaches a point of diminishing returns at 5×PQ, IIRC.
Total income may or may not also hit a point of diminishing returns at a total per turn income of 30000 bc. This is mentioned in the Economy Undocs. The ability Crime was intended to partially offset this, but it doesn't seem to be implimented yet. Whether that means the Income dropoff itself has not been implimented yet or not in unknown, at least to me.
At one time, there was consideration on making starbase upkeep exponential instead of linear, as it is now, but apparently that didn't go through.
~SDC~