When it comes to making money with android or even iOS applications, the quickest way is to purchase an already established App.

I use Flippa to purchase online businesses in the same manner but they do also offer an Apps section.

As long as you do your research into whether the Apps are truly profitable and worth what you pay for them then it can be a great way to get some extra cash.

Once again, make sure you really do your research into what you are buying, whether it is actually profitable as is, will it stay profitable long enough to get a ROI or can you modify it to make it more profitable/profitable for a longer time.