Yvona, I don't think any answer would suffice as the questiion 'why is it different in you country?' remains anyway.

I can answer from the perspective of math education: it is not just 'time' and 'skill'.

It is 'responsibity' 'teacher's ability to enjoy lessons' and 'teacher's efforts'. They vary significantly depending on what, hy and to whom you teach: up to that there are such students that you would apy yourself for the right to teach them:)

"Time' also is your time you spend on 1) preparations 2) preparing, then checking, then analyzing, then giving feedback on homework.

Of course everything is different with langauge education, besides, TOEFL is always TOEFL and an experienced teacher must already have a study plan. I only mean, assumtion "I pay for time and skill" is wrong.

Supply and demand. Any teacher worth his salt knows how to teach basic grammar and conversation, but only some teachers know how to prepare students for exams. Hence the higher prices for exam prep. 

Irena stole my answer :) But I'll add a bit more. Think of it this way: Why does a cardiac surgeon earn more than a general practice doctor? The answer is self-evident. 

Thanks for your explanation, Irena. It makes sence, it is obvious reason for any market.  It is also the feature I do not like about italki, its uncertainty, when different teachers have different fees, which they are eligible to raise/change whenever they want and their fees do not reflect their proficiency and experience sometimes.