"When was the last time you have been to a supermarket?" DeGeneres asks.
"A long time ago," admits Gates.
" ... This will be interesting," deadpans DeGeneres.
The challenge for Gates was to guess the price of three out of five products within a dollar of their recommended retail price.
If he achieved his goal "the audience will get something", announces DeGeneres, which was met with rapturous applause from the crowd.
Gates got off to a jaw-dropping start by guessing a box of Rice-A-Roni cost $US5.
It costs $US1.
"What a bargain! I'll take five," jokes the billionaire awkwardly.
Next up was Tide dishwashing pods. Gates guesses $US4 and then changes his answer to $US10 when the audience gasps.
They cost $US19.97.
"It's expensive to do laundry," Ellen explains.
Things look up for Gates when a package of dental floss is revealed as the next item. Gates is an avid flosser it turns out, so he nails this one by guessing $US4 (it costs $US3.78).
But the success is short-lived. The Silicone Valley king really comes unstuck when faced with a bag of Totino's pizza rolls.
He guesses $US22, which he quickly changes to $US8 after the audience almost falls off their chairs in shock.
They cost $US8.98.
Finally, he is presented with TGI Fridays frozen spinach and artichoke dip. When the audience indicates they think it costs less than $US10, Gates is surprised.
By this point Gates has visibly given up.
Twitter users around the US thoroughly enjoyed the episode:
