That's pretty awful from the user point of view though. It's bad enough to have to pay - much worse to pay a randomly fluctuating amount and feel like you need to count your pennies every time you add a feed to your reader.
Definitely would not advise this model for a feed reader.
You could charge users a fee per feed proportional to server cost (e.g., frequency of posts) and inversely proportional to the number of subscribers.
* Unpopular/infrequent feeds (like my friends' blogs) would be free
* Popular/infrequent and unpopular/frequent feeds would be cheap, maybe $1/year/user/feed
* Popular/frequent feeds would cost more, maybe $10/year/user/feed
This way you can peg your income to an exact multiple of your costs.