I think anybody taking the $500 for 3 days training hotel staff on how mystery shopping/audits work is going to be kicking themselves.
2.5 days on site working at least 25 hrs having to commit to three of them within a 6-week time frame seems a little bit much. Yes they're paying for travel but they're not paying for your time and you're getting your hotel and meals but I can't see this being worth it because you're not going to be able to enjoy anything and you can't do anything else while you're there because your client facing the whole time.