I'm really sorry to hear you got caught up with these characters. This sounds like your first experience with the infamous title loan / payday loan industry. Most people are well aware that these places charge upwards of 300% interest on the money they loan. Modern day legal loan sharks. The whole reason they exist is that they'll loan you money on the spot, when no legitimate lender will. As you found out, you must pay dearly for that convenience. Sadly, the process is legal in the states where these loans are allowed.
The bottom line is that you signed a loan contract for a ridiculously high interest rate without reading and understanding it.
"he told me that my payments would be around 660.00 a month but would be paid off in a about 3 months."
This should have been a clue. That adds up to less than you borrowed. You have to suspect that this place doesn't give away money for free.
""Who in the heck would sign up for a "principal only" loan with payments as high as rent?"
I'll bet if you go back and carefully read your loan contract, you'll find that you did.
It's very likely a legal contract and there's not much you'll be able to do about it. Payday and title loan companies are usually very good about making sure that their contracts are solid.