In my opinion, there are quite a lot of teachers/tutors who offer lessons at a very low rate which is of course nice for the students but really annoying for the teachers who work for higher wages. I still don't mind asking 10 USD for a session with no preparation on my side, but after 3 hours of active talking+guiding the conversation+correcting mistakes I start feeling that it's not enough for that amount of efforts. Most students don't realize how much concentration and nervous energy a teacher needs for a mere 'informal' tutoring session. Maybe not everyone is like me, but I'm sure I'm not the only one who's feeling like this. And when I'm thinking about those teachers who come from the UK or any other 'expensive' countries where prices are 10 times higher than in Russia or Asia, and sho still work for 15 USD/hour, that's heroic! I know most of us here are really passionate about teaching and money isn't the only thing we care about, but it's still our job (and for many it's a 'staple' one!).
So, those teachers who charge higher prices really value their time and probably they're not cornered and dispose of other ways to earn their lives. Please take into consideration these criteria when judging the teachers here.