Never paid for any stuff, there's enough up there for free.
For volleys i have to credit Brent able. For Serve Ian from eesetialtennis had a few great eye openers. F
YB has good overall content,but it takes more time.
Jeff Salzenstein, like Brent Abel, has usefull short footage on a lot of things. The Lock and Roll guy's forehand stuff is second to none (from what I have seen).