See first post for details, but I calculated a Bare Minimum Estimate cost to me: $5/month, $60/year. (per player)
Or looking at it a different way: $2.50/letter, twice a month.
Would you pay that much to try a play-by-mail game? ;p
(... also I basically have not been thinking about $ stuff until now, but all the design stuff is kinda ephemeral and I don't know really how to put it all down!)