http://aws.amazon.com/cloudfront/pricing/ the reserved capacity pricing is much better than the on demand pricing. Basically like EC2 on demand vs reservations. We set our reserved capacity at about 70-80% of what we expect to use most of the time. We could probably shave a few tenths of a cent per gig off but we get a good price on everything above what we've reserved so it's worked out.

If you use a lot of cloudfront bandwidth without setting up a reservation, yeah... you're gonna pay through the nose.