I don't think it costs the buyer that much, it's the seller who has to pay a commission for them setting up the sale. Matt Millin has a good reputation, can't remember what his site is called, right horse or something like that
I have been asked numerous time to source horses for people, i charged 10% of cost of horses and a flat rate of £30 per day i was out looking and travel costs.
Not sure how those that do it all the time charge but that might give you an idea
