I used WPI and they were fine.
Just remember, though, that they can only do part of the job. *Somebody* at the store still has to assemble and edit claims to submit, post credit memos, help them with corrections/rejects, file the paperwork, pull warranty parts, etc etc. So....start planning for all that NOW !
In our case nobody realized just how must the "warranty guy" did until he was gone. I reckon a lot of us have realized the same thing *after* somebody leaves the store
As to your specific question, I can't remember what they charged but it was about the same as we were paying for an on-site admin. We eventually went back to a full time warranty admin.
Cheers
DD