The least expensive one I know of is the Voyager 17 SLR from Hinterberg. Another option is to buy a Janome 1600p, Juki TL98, Brother PQ1500 or similar type machine that has a Quilter's Cruise Control available & send it to Wowquilts to be stretched. You might also want to check out the Bailey Home Quilter & see if there is a stitch regulator made for it. Yet another idea is a Nolting Fun Quilter or a used Nolting Hobby Quilter that you can have the stitch regulator added to. Stitch regulators cost upwards from $500 and you get what you pay for.
There are so many makes & models available now that it's very hard to do a price comparison. My best advice is to set your budget and research everything that falls into that price category then look at the next highest category and try to snag a bargain.
Do you plan to put this machine on a frame? Most of the ones I mentioned above are for machine quilting frames but can be adapted to sit inside of a table plus there are some table only models that I did not mention.
Research, research, and more research and if at all possible; attend a large quilt show so you can get some hands-on experience and ask questions of the experts.