I think it exists, as Mazen points out, they were video camera happy back in the early days. I also read that Kevin's planning to use it for leverage in the divorce, asking Britney for 30 million and possession of the kids so that the tape will never see the light of day. I've also seen that he's been offered 50 million for the tape, not 50 thousand. Source is
here, which should probably be taken with a grain of salt, but nonetheless is interesting.
I love how he's commiting extortion and the douche is too dumb to know it. Oh well, prison time will be good for his cred in the rap game, I guess.