There just isn't a "one size fits all" answer to this. The cost for longarmming a quilt varies from quilter to quilter. A lot depends on what type of quilting you want to see on your quilt. The price can also depend on the geographic area you live in. I live in the Denver metro area, and most of the LAers in my guild charge by the square inch for the quilting. So, the bigger the top, the more expensive the quilting. The quilting price per square inch loosely ranges from 1 1/2 cents per square inch to 3 cents per square inch for a medium density edge to edge. But this is a ballpark figure and it truly depends on the quilter. If a complex quilting pattern or heirloom type quilting is done, the price can be more. Quilters also usually have a minimum charge for small quilts. Some quilter charge a flat rate and some charge by the square yard and some charge by the hour.
As far as the batting, whether the batting is included as part of the quilting is (again) up to the quilter. Binding - if you want to pay to have it done- is again another cost. If the quilter has to piece the back, that can generate another charge. Some quilters assess a thread charge. You can expect to have your quilt trimmed to the top (if that's what you want done).
Turnaround times depend on the backlog of the quilter. The time of year can also impact how quickly you get your quilt back. The heaviest times for many of us are April/May (weddings and graduations) and late fall (Christmas quilts). Personally, I don't like to have a quilt in my studio longer than two weeks. I generally get them loaded and quilted within a week.
A little unsolicited advice is to interview a quilter before you hire someone and look at samples of his or her completed quilting. The individual quilter will tell you if the quilting is done per inch, per yard, or whatever. If the quilter contracts her services through a quilt shop, there will most likely be a sample of a completed quilt he/she has done at the shop. Get referrals from friends. You will probably be asked to sign a contract and the contract should specify exactly how much the work will be and will have a completion time the quilter commits to.
You asked some great questions about what to expect from a LAer. I hope this helped answer some of your questions.