I'd guess around $400 on my first 3 towns. I swore I wouldn't put any money in the 2nd one, but I did. I double swore not to put any money into the third...
I had just started my first town when the 2014(?) Treehouse of Horror launched. I needed kids to Trick-or-Treat and I had... Lisa. Then I remembered that iTunes credit that had been sitting there for years - I started the 2nd game trying to move my town to an Apple device, then continued to play it "just to see how a free game would stack up against a Premium game".
The 3rd town was started to answer a question in the Answers Forum. "It'll only take me a couple of hours to get the answer...".
My 4th town I started for research purposes. Part of the research was to find out how quickly you could get to the point where the donuts start rolling in. Then the "Ooops!" with the Burning Bushes hit and I bought hundreds so I could speed up my primary research: the Yearbook.
So I started a 5th game... It's been one thing after another what with the move to Texas. I'm slowly progressing toward Donut-ancial Security, but it's a slog. I had to quit playing for a bit in the middle of an event and use a bunch of Donuts I'd saved to catch back up because of the move. So I'm guessing another 3 months at my current rate of progress.
The thing about House Farms: Once you've pushed your Multiplier up you can sell them.