You can make some serious coin in this biz. But you face two challenges (and they aren't what you think).
1. Liberals. Since you are making serious coin and your customers are poor folks and you are helping them out, the liberals hate you. Generally not a problem by itself, but they pass laws limiting how much you can charge (fees and interest). If you are still in business then they just make payday lending illegal and you have to close up shop.
Which brings up problem #2:
2. Closing up shop. While you have an ongoing operation, most of your biz is repeat customers. They know they can rip you off - once. Then they cannot borrow again. So most folks pay you back. But once the word is out that you are closing up shop, they're not coming back to pay you.
Moving thread to CC.