Andre Roberson -- the guy who just signed a $30 MILLION contract with the OKC Thunder -- left a $14 tip on a nearly $500 restaurant bill ... just hours after inking his new deal.

For real.

Here's what we know ... Roberson and 12 of his friends hit up Chupacabra Cantina in Austin, TX on Wednesday -- downing vodka Red Bulls, vodka sodas and Long Islands. 

We're told Fly Dre and his crew seemed to be having a great time while rackin' up the $487.13 tab.

Problem is ... when time came to collect the check, dude left just $13.97 ... that's less than 3%.

Also, Roberson was trying to make the final bill an even $500 -- but miscalculated the math (it's okay, it happens).

Roberson went to social media to plead his case ... claiming he didn't feel like he needed to leave a tip at all because he only bought 1 bottle and "there was no service."