It depends a lot on your location. If there are quilt shops near you, you should be able to get some brochures with prices. You can also ask at your quilt guild if you belong to one. There is a big cost difference between an all over meander & a custom job.
Plan on spending a bit of time when you drop the top off to discuss what you want done. Ask to see their work. Most professional quilters will have some sort of standard written agreement. If they don't, insist on something in writing stating the size of the top, batting, thread, type of quilting & cost per inch/hour and estimated completion date.
The back needs to be larger than the top so contact them in advance to get their size requirements. Also, find out how they want the backing prepared. Some will request all seams be horizontal, some don't care, some will accept sheets, some won't...