Isn't it probably the case, generally, that those who would instruct for free deserve to be paid the same rate that another instructor, who is instructing primarily as a means of hours-building?
Given also that the "free" instructor is by definition doing it for fun, and would therefore presumably be enthusastic & outgoing.
Not to say that the hourbuilding instructors are doing a worse job you understand; merely to point out that if an instructor is doing a good job for free, he/she deserves to get the market going rate.
And if said instructor wants to help those less well off financially, surely there are schemes, like flying scholarships/air cadets/etc that are there to do this?