Nelly says he got way more than a quarter-pound of cheddar from Taylor Swift in exchange for a last-minute performance ... revealing she handed over $50K in a McDonald's bag!

The rapper told the story on an episode of "The Breakfast Club" earlier this week ... explaining T-Swift called him out of the blue in 2011 and told him she needed a big favor.

He says Taylor told him she and her friends had made up a routine to "Hot in Herre" the night before a concert in Houston, Texas, and asked him to fly down from St. Louis for the show.

She also offered him a ride on her own private jet ... and a Presidential Suite stay at the Four Seasons.

For whatever reason, Nelly asked for the money in a McDonald's bag ... and he says her manager -- Robert Allen -- complied.

Nelly and Taylor have been friends for years ... with the former star regularly popping up to perform at the latter's shows. They're so close that Nelly even attended her wedding to Travis Kelce in New York City with his wife Ashanti back in July.

A bag inside a bag ... want fries with that, too?