More than half his body's covered in tats ... which can cost a FORTUNE ... but NBA stud Chris Andersen tells TMZ Sports he hasn't paid a single cent for any of his artwork.
So, how did Birdman get the low, low price of "on the house" when it comes to his ink?
"I'm a walking billboard," Andersen says.
We're guess what he means is ... tattoo artists LOVE having their work displayed on a 6'10", 245-pound canvas that gets seen by millions and millions of people every week.
Translation -- it pays to be famous ... so you don't have to pay.