Photo: B-Real From Cypress Hill Rolled A Joint Worth $1,500

Cypress Hill M.C. B-Real proved he's insane in the brain last night -- by rolling a joint filled with 6 OUNCES of weed yesterday ... and then taking a photo to prove it!!

And now it's time for some SUPER FUNTIME POT MATH!

According to our amateur marijuana experts, an ounce usually runs around $300 ... so 6 ounces would be roughly $1,800.

BUT ... we're told people who buy in bulk usually get a discount -- like, "buy 5 ounces, get the 6th ounce free" kinda stuff ... so let's go ahead and call it $1,500.

TMZ