I don't really have much experience in the way of sales and marketing, but my integration efforts so far have just been to setup Pushover applications with icons ahead of time and then e-mail admins with the API key and some explanation so they don't have to do much work. I did the Github integration and Adium plugin myself to increase visibility.
I went with $3.99 because Prowl is $2.99 and Notify My Android is $3.99. I'm not convinced that a yearly service is worth the overhead and I personally wouldn't pay to use a notification app that had a yearly fee.
The bandwidth and server overhead for running such a service are pretty low and iOS/Android push notifications are free, so I'm not really worried about monthly costs for normal users outweighing the revenue the app brings in. Right now Pushover is running on one of my servers that does other things, so it's not even really costing anything tangible. If costs start to go up, I can always increase the price of the apps to slow growth (or increase revenue if it doesn't slow growth).