mystery buyer looking to purchase Kim Kardashian's sex tape needs to muster up at least $30 million if they want a CHANCE

The mystery buyer looking to purchase Kim Kardashian's sex tape needs to muster up at least $30 million if they want a CHANCE at taking it off the market ... TMZ has learned.

We spoke with Vivid Entertainment top banana Steve Hirsch ... who tells us he HAS made contact with the lawyer who's representing the mystery party interested in buying the rights to the sex tape ... and he's open to the possibility of a deal.

Hirsch tells us ... "Based on its long term value, it looks like $30 million would be a starting point for a discussion on all of the rights."

Hirsch adds, "I have no idea who is behind this offer ... but If it's Kim, I have a tremendous amount of respect for her. She has my number and can call me any time."