He's not blowing them up.
Just read this Village Voice piece about the "hippie crack" racket. I had no idea nitrous oxide was called "hippie crack," that there's something called a Nitrous Mafia, or that the whole thing was so serious:
With the ability to fill up to 350 [nitrous oxide] balloons per tank, which they sell for $5 and $10, [dealers] can bank more than $300,000 per festival, minus expenses.
It's like The Wire, but with way more giggling. The piece is worth a read.