The value of money changes over time. According to a website calculater I found, $50 in 1973, the year the book was published, is the same as nearly $250 today. That's a pretty big bet to eat fifteen worms in fifteen days. And just to help your appetite, here's what Billy's first "python" might look like:
Ah! Just kidding!
Whew! That's more like it ... but still pretty gross.
Finally, here's a picture of Billy's ultimate motivation. The minibike he wants to buy.