When you're so rich you can justify purchasing just about anything no matter how rediculous it is
Look at what that guy just bought, he must have stupid money
When one is said to have stupid money, it means they have lots of money that they frequently spend either foolishly or impulsively on objects with little or no practical function.
For example paying far more for something because it contains a popular brand logo, when a higher quality product is available for a much lower price.
Can also be used in a more extreme sense to refer to spending of money simply for the sake of spending it. Such as Bam Margera purchasing a new Hummer and intentionally crashing it hours later.
"You should get Beats headphones by Dre."
"Dude, I don't have stupid money like you do."
Bam Margera buys a new house for his parents every year. That's just stupid money.