Current Dilemma: The gross side of service commissions: sales or profit?

Our star salesman is the best closer I’ve ever seen. We pay him a salary plus commission on gross product sales. He has some pricing latitude. I’ve noticed a fairly stable gross profit percentage on products, but it’s different for services. He’s practically giving away services to get more product sales.

Service costs are somewhat vague and hard to accurately measure, but I need to profitably grow the service side of our business. Should I switch his commission structure to a gross profit percentage on services? What do your readers think?

– Jack, Sales Director