I own a 15" Bailey, purchased for less than $3000. No frame included, no stitch regulator ( though one's available) and no computorized anything...strictly manual. But it gets the job done beautifully!
I think the larger longarms are more of an industrial machine, whereas the midarms are basically stretched domestics. Also, those longarms come with their own amazing specialty frames...most worth thousands on their own. Add to that the lifetime warrantee and service support ( next to none for the Bailey, you learn from other owners and use a regular repair person). Many of the pricier machines also have computorized pattern and robotics capabilities which can be added to. Mine doesn't even have an up/ down needle function. But l love my machine...since l'm retired, and quilt as a hobby for charity and fun, it's all l can afford and need.