IMO it depends what she does for you. If she is actually using her free time to search classifieds, look around and putting the word out to people she knows that are "in the know" then yeah I would happily pay a fee. But I doubt very much she would put much effort into looking for you. If she was doing the above I think 10% is ok, but asking for 10% just to go look at a horse is a bit rich (pardon the pun). Spos that also depends on what your looking at paying? I doubt she would be satified with 10% if you found a horse for $500. Surely if this is your second horse you MUST have SOME idea of what you need, whats good and whats not?!? My vet was going to charge me $300 to come out check ALL conformation, teeth, hearing, vision, fitness, heart and gut sounds stuff, bloods and urine tests! They can also do x-rays for ~ $50 a pop. What is your "coach" going to do for her $1000 if you decide on a 10k horse?
It's up to you :) If you feel way more at easy with her opinion and O.K. Then it's worth it isn't it