To be honest, I have never seen rates so high in Prague either. Very good offer for me would be 42 USD/hour for longer term project, 35 being more standard (yet still good). I currently live outside of Prague (regional city) and salary of employed developer (I know, not fair comparison) here are less than 14 USD/hour. Actually when I claimed I would not work for that much, I was told everyone wished for that kind of salary.
I should really look into what is to be known to get 100 USD/hour for medium term projects in Prague. I would be able to live from 2 to 3 month project for full year.
Employment is a whole different world because of various compulsory benefits, compensations for time not worked, much higher taxation etc.
You have to transform yourself into a consultant, being a contractor locks you into the rates you mentioned (the ceiling is around $50 to $70; big agencies are able to sell contractors to enterprises for around $80). It's not for everyone though, the rate is higher for a reason, you have to do a lot more things yourself and many devs don't enjoy doing them. Narrowing you specialization helps very much; it's also helpful to choose only a few (around 2) verticals (e.g. logistics, material production, ...) that you work in and mention that to everyone.
I should really look into what is to be known to get 100 USD/hour for medium term projects in Prague. I would be able to live from 2 to 3 month project for full year.