As a consumer I have no issue whatsoever with 10% mark-up. As others have said you are responsible for the quality of the work and your reputation suffers when your sub performs poorly. I assume you would pay your sub once he completes his work to your satisfaction. If so, your time is valuable since you have to schedule the sub, inspect the work, process the invoice, and then invoice the owner. These tasks are by no means difficult but they are extremely time consuming. If the project is $1000, with your 10% mark-up you get paid $100. What is that, an hours time and a tank of fuel? Certainly not unreasonable to me, I think even 15% mark-up would be appropriate depending on how difficult it is to schedule and meet with the sub to go over the scope.