You should have the option of free mentoring in return for free mentorship. For example, I'm proficient enough in JavaScript, ruby, and c# to mentor others and even charge for it. But if its not a very intense schedule, I'd happy to do it for free in return for free mentorship of something I am currently learning (such as Haskell or iOS development). So if I get credits for free mentoring that I can use to pay other mentors, that would be sweet. Though i suppose I might as well just charge for the mentoring I do and pay other mentors for that money.