Nate Diaz's boxing coach says his fighter needs BIG money ... or else he ain't coming back to fight Conor McGregor a third time.

How much? 

"At least $20 million, $30 million,” Richard Perez told Submission Radio. "Come on. UFC’s making a whole lot of money."

"They’re giving more to McGregor, so it’s not fair because it takes two in that ring to draw a crowd."

Conor's been droppin' hints that a Diaz trilogy is what he wants for his UFC return -- basically challenging Nate to do the dance at 155 pounds for the belt. 

Diaz seems down ... but clearly feels he deserves just as much scratch as his superstar rival. 

There's more ... Perez says Nate will whoop on Conor if he gets through training camp uninjured -- which he says wasn't the case for their UFC 202 rematch -- and guarantees his boy is the better boxer of the two.

Shots fired.