Playas in town for Mayweather vs. McGregor are blowin' their wads in a BIG way -- droppin' $30-40K on sex for the Vegas super-fight ... so says Don "Magic" Juan

We got The Bishop -- a notorious ex-pimp who's also Snoop's spiritual adviser -- outside MGM Grand ... and he told us the ladies are charging BANK for their services this weekend.

"Floyd -- he draw them money-gettin' chicks .. they chargin' that paper for real."

As for how much ... Magic estimates about 15 RACKS for an overnight -- and even more for the VIP treatment.

Basically, Sin City's turned into Hookerpalooza ... and DMJ says it's even crazier than he thought it'd be.

"Ladies and playas and alcohol and partying -- all that go together. It's goin' down tonight!"

Sounds about right.