How much to expect to pay a good SEO company

Good evening everyone and experts, I am recently in the market for a SEO company. I am 100% looking for quality and a good ROI, and I do understand SEO takes time. My two questions would be: how much should I expect to pay a good SEO company, and how would you personally vet out a company to see if they are the real deal and not some beginner who took a few courses (no offense to anyone who is a beginner who took a few courses) Thank you! I have a service business as well.