I'm struggling to understand why it's a 'service' at all, as opposed an app that can be purchased, like say http://panic.com/statusboard/
Maybe we are not communicating the product well on the site.
It's OK to say "we don't think enough people will pay enough for a one-time purchase to cover development costs". You don't have to invent some kind of technical reason for it to be a service.
But having said that, I cannot believe that companies would pay the amount you're asking for a service either.