UFC's Brittney Palmer -- James Dean Painting ... Sells for $3,000

Brittney Palmer's fascination with James Dean just paid off -- the UFC ring girl just sold a painting she made of the Hollywood icon for $3,000 ... TMZ Sports has learned.

As we previously reported -- Palmer's career outside the ring began to take off ... with powerful people like Dana White and computer mogul Michael Dell owning some of her work.

Now, we've learned Palmer's latest creation was also snatched up rather quickly -- with an art enthusiast dropping $3k on the piece after seeing it on Instagram.

No word on the identity of the buyer ... but a source close to the situation tell us, "the person is a big fan of Brittney."