Mock Rounds can be quite useful for candidates and they might be willing to pay for it too. But harder problem is supply of professionals. This works well when there is wide range of professionals conducting these rounds. Most professionals (good ones) are tied to their jobs, make good money and would rather spend and hour with family or friends than conducting an interview that pays less than 50$ or so.

You can check out tripple byte, although not similar model but they seem to have figured out supply side well, mostly due to its association with YC.