My understanding is this:
When a player gets an Ace on the first card in blackjack, they usually ask for a picture or 10 on the second card. Sometimes people would snap their finger or hit the table or yell snap or monkey or something else to get a 10 value to get blackjac. That was the origin of the snap.
In Atlantic City all casinos had to have standard denominations of $1, 2.50, 5, 25, 100 etc. The $2.50 was there to pay off $5 blackjacks. Thus it became the snapper.
Believe it or not.