Honestly - it totally depends upon the area of the country (there are many different price points, depending upon what the going rate is in the area) and also the design you choose. Most people charge by the inch, but you will find some that will do an allover meander type quilting pattern for a set price per size.
A local person in your area can work with you and show you many different ways you could quilt a quilt on a budget, and you have the advantage of not loosing the quilt in shipping - and assurance you get what you wanted. This is only my opinion...and experience :) Good luck!