As I wrote, a worthy goal unto itself but not "sitting on a goldmine".

Not idiomatically, but there are plenty of gold mining operations that produced less than half an ounce of gold per month on average.

Right now I'm working on a little something of my own, and if I can reach 500$ a month that will almost cover half of my college tuition :) (state school)

That would nearly cover all my expenses, including rent, food, and tuition.