There are no ads in the game, there is no pay-to-win. Devs don't do this out of the kindness of their hearts. It's their job and they need to get paid.
The fact that you get the entire base game for free is already an amazing deal. If you are, in-fact, 18 years old, it should be pretty easy to scrape together $10.
Dev works hard, makes an amazing product and deserves to be paid for his efforts.