I have paid any where between $35. for a small one up to $112. for a king size. I furnished the back and batting. Depends on where you live etc.

location matters- and the quilter-
some charge by the project- some by the square inch---and it varies greatly-
there is a list of long-arm quilters here on the board- check out the list= contact a few and ask how much (they charge)
look at different one's work- and choose people who quilt in ways you like-
some people will quilt a queen size quilt for $50 or even less- some a lot more-
lots of variables--the first quilt i had professionally quilted cost me $385---i was so unprepared...
it varies greatly state to state- town to town- and experience of the quilter....along with the type of quilting you want done...check out the list and see if there is anyone in your 'neck of the woods'
some do (long-distance)quilting too-

Bits n pieces has a flat charge...think is 90.00 for that size. They do an incredible job...I will look for the price sheet. I found it...the price is at the bottom of their page. http://www.bnpquilts.com/customquilting.html

Twin 65 x 90 $ 85.00 Full Size 84 x 94 $ 90.00

They charge you about the same if you do it or if they do it. It is easier and faster if they do it.

there is another quilt shop that does twin sizes for 48.00 not sure what the charge is for the others. I have that posted some where...I will see if I can find it.