You have to know beforehand, is the mentor teaching you voluntarily or is there something else he wants to get from you as a token of service? Of course, in this case you understand the mentor's side much better because usually people will be seen when we start to go deeper into their lives.
Mostly there will be a prior agreement and if the mentor is willing to teach voluntarily, I think there should also be appropriate compensation when in the future we have absorbed a lot of knowledge from him, and it all comes back to our conscience.
In fact, all knowledge cannot be measured with money, but there are other things we can do to repay kindness to the people who have taught us.
Money is actually the main reason why mentors accept newbies for mentoring. But we should also know that if we are grateful for what they have done to us, we can always chose to give them more as long as its given through goodwill.
However, there are also those who don't. I once found a mentor who taught many people without any reward, but he got more than he should.
That is proof and the result of sincerity which is rewarded in a more beautiful way.
And personally, I also don't like a mentor who teaches because he only thinks about money, because we can't teach everyone and then succeed.