How much to charge for referrals

I recently purchased an investment property outside my area and I have other fellow investors interested in that area. The investor I purchased from is excellent and I see a (hopefully) long term relationship with him. I would like to refer the investors I know to him and he has agreed to pay a commission or a flat fee per deal he closes with my referrals. I would like the opinions of you investors as to what would be a good commission or flat fee agreement that would be fair to both myself and the other investor. Is there a norm? I don’t want to ask for too much and appear greedy but I also don’t want to cheat myself as I am doing this as a full time business. Thanks in advance for any help : )

It depend on the selling price of the home.