Rob,
Think of the commision as an advertising cost - if you are currently spending X% of sales as the cost of getting work, then that would be a starting point for the commision percentage.
You may wish to add to this if you feel that the overall value to your business of antisipated extra sales warrants the cost.
On the other hand they will be looking for the best cut they can get, so the final figure will be the result of negotiation.
John.