It is expensive. However, some women enjoy the fun of a monthly package coming. Some of the shops do an extraordinary job of kitting up these BOMS and they mostly have fabulous fabric. I love having hundreds of different fabrics in my quilts and this is the easiest way to do it. I mean it is not cheap to buy 1/4 yard of all the scraps needed for a complex quilt. So there is are a number of reasons to get a kit or BOM. For some, money is not an issue.
My idea is to buy them off ebay or garage sales/estate sales once the mailing is finished. Lots of quilters change their mind about making a complex quilt or die and leave an estate, or whatever and get rid of their kits. I have made Lori Smith BOMs, oriental sampler, fat quarter shop sampler, and many others. I even got a Lynette Anderson Raining Cats and Dogs BOM.
Many quilt shops do a version where you buy the first month and then bring the finished block back next month to get the next block for free. A set of quilt blocks can be $5-20 and then you add whatever finishing kit or fabric you want to finish it up. This is fun but quite a commitment.