I don't do any mail order ones but usually do one offered at my LQS each year. First month is $20-30. Each month you bring back a completed block, that months pieces are free. In reality you can get all your blocks for your top for that one initial investment. They base their size variations (lap to king) on using sashings; extra blocks; plain blocks whatever. The 'finishing kits' are extra and the price varies predicated on size.
The primary reason I like doing this is the skill-building aspect. I'm generally not a 'sampler or scrappy' kind of person but they usually come up with either different techniques for same units (flying geese come to mind) or some other skill building exercise.
And sometimes, it works out that the colors, etc. are something that I'm planning for gifting anyhow so it all works.