That's right, the technical depth that the interviews would dive to would be different depending on the experience your candidate is expected to have.
(0.25 * base salary * (average number of broken hires - 1))
How much time would it take for an expert to do this? Not just in the interview part, but the after-service (write-ups)?
I'm sure if they tethered their prices to the value they're actually creating, and they had experts capable of screening clients, then I could see this taking off.