I'm with Doc C on this one. In most cases, I won't mix pay and pleasure. As soon as I start to do something for somebody else's money, it becomes work. I think building a model for your own enjoyment then selling it is different than building one on commission. For me, the quickest way to kill the fun is to change a "want to" into a "have to."
But that's my opinion. There are folks for whom this situation works, and works quite well. I'm just not one of them.
David G.