Maybe if Google didn't fail pricing 101, they'd know something like 18.25% is more palatable to a consumer than a big round number like 20.
In this case, there are other forums and other ways to get "helpouts" not to mention if it really does take off, expect other major sites to launch their own helpouts efforts and ignore Google's offerings.
I am a math teacher, and I've seen math teachers make $80+/hr tutoring high school and college students. I'd consider trying this platform out, because it would take little effort to get started, and I'd have nothing to lose.
I'd be a little frustrated to see Google take 20% right off the top. But I am assuming that Google has the infrastructure to roll this out without needing to take 20% to maintain profitability on this. Does anyone have a sense of a reasonable fee for this service?