Uh, there were 21 D3 X-Ball teams in So Cal. That's sold out? There were 6 D1 X-Ball teams. I know that isn't sold out.
There were 66 D3 7-man teams in Boston, by comparison, and 35 D1 teams.
I don't recall saying that PSP sold out all their events - just that price wasn't the obstacle. SoCal didn't sell out for other reasons - last minute location confirmation, expense of flying into LA, and the lack of expense of flying into Charlotte coupled with the ability to drop one of the 6 scores all contributed.
As for the costs per team, X-Ball involves more hotel, rental car, paint and airfare than 7-man. And any other expensive that comes down to a per-player cost (stipends or even team dinners) are going to be more expensive for X-Ball than for 7-man.
I'm surprised any reasonable person would debate this point.
I specified D3. D3 and 7-man roster sizes are pretty much the same. D3 caps at 10 players, same as 7-man. If you had players that excercised, you could even get away with playing D3 with 6 bodies (not that I'd recommend it).
There is more paint shot, but the entry fee is lower.
Now, D2 XBall, on a team basis, is more expensive than 7-man. But, all formats of XBall provide a LOT more play time than 7-man, and are a LOT better at picking the appropriate winner.
It is a mistake to assume that the only criteria players use when deciding what format to play is price. It is a double mistake to assume players are picking a format based on price when that format isn't even significantly less expensive than the alternative.