I build frames as my full time job, and don't make squat. I work hard, and make enough to pay the bills, but thats it. I am not getting rich by any stretch of the imagination. The only decent money I make is selling forks and T shirts with my frames.
The margin and the $$ will not come from building and selling the frame. The custom frame will bring a customer in, but to make a living out of it is important to have other things the customer is interested in buying after bringing them in for a custom frame.
But I have only been doing this for half a year, so it is probably a different story for people who have been building for a while. They build faster, build better, and sell for more.