Short Sale - Negotiator Fee

[Estate Vestor]

I have been negotiaing with banks on Short Sales and Modifications but never on a fee for service basis. I have someone who asked me to negotiate a Short Sale for them. Property is already listed with the Realtor and they have an offer, the owner wants me to do the negotiation.
How much is a typical fee that negotiators charge? And, does BofA typically agree to pay negotiator fee in addition to commissions.