p.p.s. I've got a lot of other ideas too but I'm tired of typing. Suffice it to say there are all types of ways to structure something like this and you need to be careful to keep as much control as possible and make the builder need YOU as much as possible. The beauty of it is that all the builder is going to care about is his cut. If you do it well he is no longer going to be concerned with whether you are the lowest price because the client is buying the upgrade options. I would also create options you could demo to the clients at YOUR showroom. Can the electrician do that?