My friend did an extremely simple Unity game (with nice graphics and music), added AdMob advertising, put an Android version as a free game on Google Play, and gets about 20 dollars a month (during the recent half of the year, and the number seems stable). That's the only data point I have.

I suppose your game would be better (but I don't really know what the players value), so... let's make a wild guess that it could make 50 dollars a month during the following 5 years. That's like 5×12×50 = 3000 dollars total. Maybe! If you need 9 months to finish it (beware the planning fallacy!), it is 300 dollars per month of work. I don't know how much time during the month you would spend coding. Discounting for the planning fallacy and the uncertainty of outcome, let's make it, say, 100 dollars per month of work.

Of course, besides money you get some additional benefits such as feeling good and having a nice item in your portfolio (probably irrelevant for most jobs you consider).