Loup has a bit of a point, but so does the OP, even though I really can't give much credence to a plea for realism in a highly abstract game like GC2.
Smacksay, your frustration is not news to longtime forum folks. Many of us have had similar frustrating moments when we encountered one of the hard-coded econ oddities. The treasury >10k and >20k penalties get more attention, maybe because most folks are just 'naturally' averse to deficit spending. What GC3 needs is a model that links these values to a specific game context (population, number of colonies, average past 5 turns income, whatever). When you're taking in many thousands of BCs per turn, the -500 debt floor just seems silly. Just keep in mind that there are many players who prefer small, short games, and on those maps 500 BC tends to remain 'a fortune.'