Eight farms at ten times the price of the last, an office to upgrade, and a sell button. Buy farms, boost them, sell the product, buy better farms. The maths is completely conventional and the theme is doing all the work.
The achievements are the joke: they run from You gotta start somewhere through Why do you play this and Do I need to get you a therapist to Stop playing and No life. The author made it as a joke and then finished it because people at school asked them to, and it plays exactly like that. An hour.