The mentoring relationship exists primarily because the mentor want to "give back". A good mentor will typically not be motivated by personal gain. (And if they are, they will likely not be a very good mentor).

I typically believe that success happens in business by providing an opportunity for gain, however I think this may be an exception.

A mentor may benefit from the contacts and potential business relationships that may be created by a successful protégé, however these are generally secondary and will not happen in the short term.