There's more to consider when justifying a quilter than the direct costs of sending the labor out instead. You would have 100% control of your quilts and you would be able to get them done in your own time frame. That can be good and bad. Longarming is a labor intensive effort and it can be frustrating sometimes. The learning curve to being excellent at it is HIGH. Make sure you will enjoy it before spending so much for it. I saw a post on some board talking about how so many people have longarms just sitting in their basement that they never used. They thought it was such a good idea, now they have to eat much of the cost to sell it so it sits.
The top of the line isn't necessarily the top of your line. And the $15,000 machine may not be right for you either. Shop around, as other people have said. There's a show in Salt Lake City in May and another in New Hampshire in April.
The mechanical aspects of quilting machines live on forever, if you oil and clean the machine well. It's the computers that die, because they seem to frequently have extra cheap circuitry, so be careful about that, especially when considering used. (And I am not a computer-phobe. I work in IT).
I have a 30 inch Gammill longarm, all mechanical, that I bought used in 1994 for $3000, and it's still a gem. The frame is amazing. All of the mechanics are exposed so there is no mystery, no sending it in to some factory for a mysterious "spa treatment". I know exactly how it works and do all maintenance myself...and the maintenance is small since I only do my quilts.
Owning a longarm for yourself is a JOY. You just need to go in with your eyes open.