If someone is mentoring you it means that they're so much greater than you that yo can't rationally explain why they would spend any of their precious time on you.

... yet they still do. How can this be explained? 

Simple: they aren't spending time on you. The mentor is getting something out of you. To them you are also greater than them in ways you can't imagine from your position given that you are someone with potential rather than actualized greatness. A true mentor is someone who sees that in you. You can't buy this.

If you're paying for 

mentorship

then your mentor is probably not your mentor but rather just another business you are helping to support. Nothing wrong with that by the way.