Today Amazon introduced "reserved instance" pricing for users who tend to run long lived instances.
The usual pricing for a small instance is $0.10 per cpu-hour, which works out to about $73/month if you keep your instance alive 24x7.
With the reserved pricing you can pay a fixed amount for a term of 1 year or 3 years and get a reduced hourly usage rate of $.03 per cpu-hour For 1 year the up front fee is $325, for 3 years it's $500.
If you run your instance all the time I believe that works out to:
1 year - $49/mo
3 year - $36/mo
And there can still be savings if you don't run your instance continuously. Note that the 1 year and 3 year pricing are 2/3 and 1/2 the monthly cost respectively. So if you run your instances more than that fraction of the time you'll still see benefit by going with the reserved pricing. i.e. if you run your instance more than 12 hours a day during a 3 year period you should pay the up front fee to save money.