Lots of variables on this!
Most of us charge by the square inch, some by the square foot. The less intensive, the less charge. The more quilting and the more specific to your quilt, the higher the charge.
To get square inches: length x width
Some of us can supply batting, which can sometimes save you money if you are shipping your top (only have to pay for the batting one way!). If you want to supply your own batting, make sure it is something that the quilter is willing to use. Not all battings are created equal. I have found one batting, Mountain Mist Poly, that I will not use. It's inconsistent, feels yucky, and I've heard it has fiberglass in it. I (and the other quilters for hire that I know) want to give back a quality product.
Make sure your back is 6-8" longer and 6-8" wider than your top, and have it square. This allows us room to attach to the machine, and have room to test tensions.
Hope this helps!