Most BOMs are actually pretty affordable. You don't pay the whole cost at once. They're generally divided into 12 or 13 months. Most of them run around $25.00 a month. I've seen a few as high as $40.00 a month. I have seen fabrics that the only way you can get the focus fabrics is thru the BOM.
Keep in mind the shop owner has to purchase the program and all the fabrics for the program up front. Then those fabrics have to be cut and packaged into individual monthly packages for however many people are participating.
Every shop owner will tell you they have lots and lots of customers that don't want to pick their own fabrics, they want a totally coordinated quilt. BOM programs are perfect for those customers.