How to judge fair pricing for CRO services?

Recently, I’ve been exploring the market for conversion rate optimization services, but honestly, I’m struggling to understand how to judge what a fair price is. It seems like every agency or consultant charges differently and it’s hard to compare offers. I’m running a mid-sized e-commerce site and want to invest wisely, but I’m unsure how to evaluate potential results against their fees. Has anyone here gone through this process and figured out practical ways to determine if a CRO service is reasonably priced? Also, do specific business types generally face higher or lower costs? I want to avoid paying too much but ensure the quality of work is there since this can really impact revenue. Any insights or personal experience would be great.