It's more like they're offering $500 to be taught how to utilise the exploit.

There’s a lot of techies on this site for whom that’s peanuts.

I'm sure OP is a good person.

But if they were not, and had ill intentions, they might see $500 as a very inexpensive way to purchase a working exploit to use against others.

It is a price signal. They expect to lose the money they want to show that nobody has an automated swarm just pwning every server running PHP connected to the internet. At least not yet, on day 5 and counting.