I have a medium-sized plot of land, and have a total population of exactly
246,063. I got this with a glitch I discovered (which only sometimes works)
1) You found a city, make a minor 2-3K population.
(You can't have any trash disposal systems or it will fail)
2) Establish a deal with two neighboring cities: one to import 10 units and the other to export the max.
3) Wait. It should be within 20-30 years before you get to $50 million.
IMPORTANT: Do the import first so the glitch will register!!!!
You then should be able to build a massive city.
(And don't grow the city too fast, or else trash will overflow.)
Good Luck!