I got my daughter a used 24" bike at Marty's for way under $500. I think I paid around $200.specialized riprock 24" or similar. Like to keep it under $500.
Without going used, that's about my upper limit too. Also looking at the Riprock 24, and I stumbled on the Orbea MX 24 Trail yesterday, but that's a bit higher. It is a 1x10 with hyd disc and an air fork. The boy is at the very lower end of fitting on a 24, so weight means a bit more.