Kylie Jenner should snatch the $10 million to do a sex tape and never look back ... because no less a XXX authority than Larry Flynt Jr. says that's way more than she's worth on the open porn market.
The Hustler king was at Craig's Thursday night ... and we asked about the offer Vivid Entertainment has on the table for the 18-year-old to go all "Kim K. Superstar."
Larry laughs off the ass-tronomical figure, but does admit there's a market for Kylie -- and floats a hypothetical offer for her to do the nasty for Hustler.
He also takes a jab at Vivid's honcho. Porn-on-porn violence ... it's gotta stop.