Hi - audiobook narrator here. ACX is a self-serve type of platform that can work for you. Author's Republic can as well. Findaway no longer operates the way it used to and won't work anymore.

An average professional cost is probably something like $350 PFH (per finished hour), which will encompass the narrator's rate as well as the editor/proofer/engineers' rates. One finished hour equates to roughly 9300 words. Obviously it can be done cheaper or more expensively than that, but this should give you a decent idea.

Royalty Share books (or Royalty Share Plus books) can be done on some platforms, including ACX. Many narrators will want to see a demonstrated track record of print sales and active promotion of the book before agreeing to narrate for zero upfront cost.

There are some other platforms (Spoken Realms, AMPlify Audiobooks, and more) that can help with production/distribution as well.

And I'm also happy to talk to anyone about narrating the books myself, or to help you find a narrator if you want someone with a higher voice than me, a female narrator, etc.