No clue if it is normal, but I do know that H1Bs cost around $6,300 excluding lawyer fees. So the $6,500 is likely their minimal outlay for applying for a successful visa, money they cannot recover if you quit a day after it is granted.

I have no clue if it is legal to try and recover the money from you. You'd have to consult a lawyer to figure that out. I am just saying that's likely where the $6,500 comes from, it isn't a "punishment" figure but rather a "recovery" figure.

I'd definitely want an ironclad contract setting out what they're paying you for the full 18 months. Otherwise they could pay you essentially nothing and hold you there "hostage." However if you're moving into a good job, and you're already getting paid an amount where the $6,500 is trivial, I'd just do it. Worst case scenario you pay them the $6,500 and be on your way.