Aretha Franklin -- The $40,000 Eat-and-Run

Aretha Franklin just left Sarah Jessica Parker's $40,000 a-plate dinner party ... after spending just 20 MINUTES INSIDE ... that's $2,000-per minute!!!!!

Aretha -- who was the first to arrive to the Barack Obama fundraiser -- was also the first to the leave.

On her way out, Ms. Franklin told us the food was impressive ... "chicken with a mustard sauce, diced tomatoes and a lot of relishes on the side of the plate."