As a professional in the industry I thought I would add my angle to this one. Of course, this is just my personal opinion.
When looking for a professional quilter, remember that you get what you pay for. If you are looking for a bargain...you need to be prepared to receive it. Most people spend countless hours choosing fabrics and patterns, then countless more hours cutting and sewing and pressing. Most people are careful to invest in the good quality fabrics found at their local quilt shop, and don't skimp on thread either. If you are one of these people, I implore you to not put "get a bargain" at the top of your priority list when searching for a machine quilter.
I would begin my search by asking friends/guild members/quilt shops for referrals. Ask about quality first. Find out who in your area is experienced, professional, and produces high quality work. Second, call a couple of the referrals and ask them their philosophy on quilting, their style, and ask if they have samples that you can see. Many of us have sample photos online, or can direct you to a quiltshop that has samples of our work hanging on the walls. Not only do you need to see that we produce quality work, but you need to find out if we quilt in the style that you prefer.
Now, on the topic of price. Most of us have a scale for our prices. For instance, I charge a whole lot less for edge-to-edge freemotion then I do for heirloom work with trapunto. Be honest with your quilter, let him/her know that you want it done at their inexpensive end of the scale. Even if it's quilted simply, if you have chosen a quality professional, you will get a quality quilt job. Believe me, there is a HUGE difference between a quality quilting job and a sloppy one, even if it's just a simple meander.
As far as wait time goes, understand that a quality professional will have a reasonable wait time. It's not that it takes us FOREVER to finish your quilt, it's that we need to give our time, love, and full attention to every single quilt ahead of yours in line. Be patient and your quilt will also get all of our time, attention, and love when it's turn is up. Also remember, that any professional with a good reputation will have a large customer base and therefore will have a longer list of quilts on their schedule.
Good luck to you!