What’s a fair buyout price in this case?

I’m currently working with a client through a another firm that “placed” me there. It’s not a contract shop; I was brought in as a subcontractor. Due to the other firm’s carelessness I have not signed a non-compete with them.

I’ve been working with this client for more than a year, staffed up the project, and grown the business.

The other form hasn’t done anything to manage the account so after a year the end client has decided that they would prefer to work directly with me.

They are willing to compensate the first firm for walking away from the relationship. As a ball-park firgure, what would be a reasonable amount?

Thanks.