Wow, it depends on so many things. The size of the quilt, the quilting pattern you choose, the part of the country you live in, if it's a rush job. So many variables. For a simple edge to edge pattern, I pay . 0133 cents per square inch (psi). Custom can go up to .05 psi. Most longarmers have their rates psi on their websires. Check our some in your area. Be sure to ask a lot of questions and throughly "interview" the person before you decide to go ahead.
Check these threads from the past
http://www.quiltingboard.com/posts/list/2940.page
http://www.quiltingboard.com/posts/list/9217.page