Joe Johnson, running on fumes, helps Brit economy!

This is the sort to thing you can do when you have a $120 million contract ... and you're in London.

TMZ reports that Joe Johnson dropped $10,000 Monday night on champagne(!) while visiting the China White nightclub in London.  (No wonder he said on Tuesday he's running on fumes!)

TMZ provides all the effervescent details...

Joe chalked up some big numbers at the nightclub ... buying 7 bottles of Dom Perignon Rose Luminous (at $1,200-a-pop), 2 shots of Sex on the Beach and some water.

Plus, Joe left a $960 tip on the bill ... making his bill for the night just shy of $10,000.

BALLER!!