The fact you refunded the deposit alone should clear you of any responsibility, both financially and work-wise. Smart move there.

Ultimately this comes down to your contract with them. You did have them agree to a contract detailing everything right??

Although some will say a contract is worth as much as the paper it's written on (or how much one is willing to pay a prosecutor) it's still smart to outline expectations from both sides before beginning work.

I'm curious though -- define "insane amount of money".

Also, what went wrong? Unmet expectations? Better offer come along? Burn out?