You represent the buyer and you're his broker so I normally would've expected the client and broker to have discussed these details when they established the relationship. I've dealt w/brokers a lot and I know your obligation is towards the buyer since he hired you (getting him the best possible price and terms)
But if I were the buyer I would not pay more than 15% no matter what the broker's work actually was. At least not in domaining. 10% is actually more common for private negotiations (it may be that your client had a good idea of what he wanted and where to buy it to begin with and he just didn't want to deal w/the seller, and in these cases brokers don't have to sort thru piles of offers, advertising, following up w/several parties, etc) but that's between you and your client. If I'm the buyer I'll keep in mind I may not be able to offer as much as I want since I'll owe a fee of up to 15%, and if when seller is paying the fee he often won't lower the price much due to that
As to what to tell your guy to offer, it's hard to tell not knowing how unique the domain is or how bad your client wants it, but rule of thumb is never more than 80% of asking price as long as the asking is fair. I'd go 75% if your client has any other choices and you feel the seller is someone you can work with. I've known sellers who own things the buyer wants badly and they simply walk away unless they get what they want at once, that's something to consider too. And if the seller has not set a price and you guys are approaching him out of the blue it's even riskier.
Any chance of getting a 50/50 split for your fee ? I've found that deals sometimes work out better that way, and in the real world it's the seller who traditionally negotiates and pays any commission. Good luck w/your deal anyway
:nod: