Kim Kardashian may have done her naked Paper Cover for free... but there's one person making bank off it - Ray J!

Yes, every time Kim gets naked, interest is triggered in her sex tape with Ray J and last week was no exception.

According to TMZ, sales of Vivid Entertainment's "Kim K Superstar" have dramatically spiked, earning Ray J a nice $50K from the last week alone.

This is a huge increase on what he usually makes from the tape, which is reportedly $90K every three months.

And it gets better - due to this recent burst of interest, Kim's sex tape is officially the best selling celebrity porno of all time, knocking Pamela Anderson and Tommy Lee off the top spot.

Gosh North West is going to be so PROUD when she's old enough to realize!