CloudFlare have been doing this to all the big image providers recently and don’t seem to understand the jump in pricing to their top tier is not remotely possible for smaller projects like ourselves. One interesting point is their free tear didn't seem to get those “upgrade” emails!
The prices are negotiable with them, but your right, its not sustainable.
I know fanart.tv moved to a dedicated server for pushing images which was about $20 a month. You don’t get the DDOS protection, but bandwidth is relatively cheap these days if you look in the right places.