Well, one idea could work like this, maybe:
A person get one advise at a time, as you said (at the time coach can answer, giving priority to those who pay a monthly fee).
Then if that person feels the coach goes in his/her direction (means actually the coach helps him/her with the advise) then the person would be highly motivated to keep the Coach consulting more often, because he/she is growing, so they will be happy to pay the Coach to work a little `closer` (obviously more email frequency).
Grow is what all of us we are looking for. And it must, in its esence, keep doing it all the time. It could be a lifetime relationship. Like any relationship has in common, that mutual benefit of grow and expand.
That would work, I think.
Specially if the Coach get enough email (Coach got famous let say) some volume of `free` emails would lead on some `monthly-payment` emails.
Coach can also give some affordable rate for the volume of jobs the internet gives them. Without compromise the quality of the coaching session that can compete effortlessly with others coachs.
That`s how sales work anyway.
Let me tell you CraigL, this is actually happen, by phone, many Coachs and Coach Companies are giving coach thru phone to distant customers for a montly fee or a prepaid fee for x times of coach advise.
Sometime people asociate that giving something for free means being stupid, how far they are from reality! isn`t it?
I do it all the time and keep me in the good track. You are doing it now also maybe without noticing it.
What I mean, I give my customer or potential customer (those `free` emails?) an advise about what I do, computers and stuff, including some other ideas no necessarily related to those.
And they are happy to get me around. So they start unconsiously need me for ...computer.
Think this is not a trap, all the opposite (sorry for my English),
I give sincerely deep advise from my heart, for free and not thinking never-ever in that INEVITABLE fruit it will give me anyway. However I get not only more jobs, I gain new friends or any type of new relationships, I gain advise also from them in other areas they are expert. I gain a lot in many ways.
Because I love to help people. So the universe give you that win-win circle.
If you think only on money, that`s only what you get, nothing else, nothing more, the challenge is not everybody is looking for the same and your circle will be small.
Sorry I got inspired like Aristoteles sometime.