I don't think I would like that at all. For me, it would make the situation feel 'cold'. I send 3-4 emails per week. That would probably cost me $100 month! I don't think that would work for me. Since I am the only client she has that emails, there has been no problem at all. She assures me that she is fine with it, but to understand that sometimes it may take her a couple of days to respond. If I put urgent in the subject line, I will hear back from her that day.

$100 for 20 emails? That would be different for me. I wouldn't feel that my emails would be as personal and emotional if I were paying to have them read.