$0.01 per hour when not mapped to a running instanceThe idea is to encourage people to stop hogging public IP addresses, which are a limited resource, when they don't need them.
I think one way of viewing EC2 - and the other Amazon utility services - is as a way of putting very fine-grained costs on various computing operations. So will such a pricing model drive us to minimise the computing resources we use to solve a particular problem? My hope is that making computing costs more transparent will at least make us think about what we're using more, in the way metered electricity makes (some of) us think twice about leaving the lights on. Perhaps we'll even start talking about optimizing for monetary cost or energy usage rather than purely raw speed?