Actually, the price of longarms has been coming down, mainly because of their popularity. There are a lot of new brands entering the market with their version, and are at a "hobby quilter" price point, not a "professional quilter" price.
The video was fascinating. I always figured they didn't make a larger bobbin than the M because of the mechanics of the stitch. The upper thread has to go completely around the bobbin in order to make the stitch, and I assumed if the bobbin was much bigger, it would throw the stitch or the tension or the timing off. Would be a good question to ask the manufacturer reps, wouldn't it?
This is exactly the reason why I like to use a fine thread, such as Bottom Line, in the bobbin. You can get a lot more 60 wt thread on a bobbin than you can a 40 wt thread!