How can I charge a "finder's fee" from competitors when I give them business?

I provide software services in a very niche market. I have built myself a name and brand recognition, and I receive a lot of client leads every day. I get more leads than I can handle so I have to reject a lot of projects.A few of the other big players in my market don't get as many clients as I do, and they have often asked me to refer some of my clients to them. In the past I've done this free of charge, but I'm understanding now that I'm cutting down their advertising costs (I don't advertise myself - I get only organic leads) and bringing them a lot of revenue.So I'd like to offer them a partnership where for every client I send them, I get a finder's fee. Each project is typically $10k-$20k. I think it's fair because I've worked very hard to get to this point where I get so many leads. There are several unknowns to me in how to approach this:Is this a common thing to do?Is this called a "finder's fee"? Is there a better term I could use when I offer this to them?Is asking for 10% of their project total and 10% of ongoing support a fair price? Is it extremely low or extremely high?Suppose we agree on 10%, how can I enforce this and know what the actual price of their contract is? Do I ask to remain on all email communications so that I now what their agreed price is, or do I ask for them to send me the signed contract once they reach a deal with the client, or do I just trust that they'll tell me the truth? see hubwealthy.com/wealthy