I haven't checkout out BOM's lately at our quilt shops but I used to participate at several different shops. Here's the way they worked:
sign up and attend at appointed time, usually before the shop opens.
The first block costs $10, you get pattern and fabric to make it. If you bring your completed block the next month and you'll get the next block free. But if you miss that day you have to pay another $10 to get the next block. I think I have 3 sets in my UFO pile. The blocks are done - I just have to put them together.