Aside from the usual issues for an online business:
1) There exist multiple classes of well-employed professionals who value this far more than almost any other users
2) ...
A wide range, actually, but when I think of that number I think $150 to $600 per hour.
"Nobody pays $600 an hour for English practice."
I mean, false.
Mostly people who can calculate the net present value of their career without needing to look up what net present value means.