For insurance credentialing, there is no cost. It's a pain, and sometimes time consuming and frustrating, but doesn't cost anything. Some practices choose to outsource that, but they certainly don't have to. It's the same work you had to do previously for any of your current providers, whether NP, PA, MD, DO, etc. The way I look at it is if you are a small group, it's not that much work. If you are a large group, with frequent changing and updating of providers, you can make it part of someone's job to do that work. And even if you outsource, your clinician still needs to have some level of involvement (signing, creating accounts, etc.)
If you are referring to hospital credentialing (to have privileges), then most hospitals charge an annual fee/dues. Around here, I've seen it range from $100 - $300. Again, same cost as your current clinicians.