When you start your game, spend ALL your money initially buying up real estate. Shops are the preferred, and of those, pawnbrokers and pubs. Around the time you get the 2m real estate magnate achievement, you'll be making at least 50k per every 5 minutes.
Also, the pie mini-game is, at least imo, the best job to make money in, especially once you get it up to the max level. If you get to multiplier 10 without failing, you'll make 3360 gold per successful completion, which takes seconds to do. Easiest way to build up quick money for your real estate empire. 200k should do it if you continually re-invest it whenever you mess up and break your multiplier chain.