For me it's CPU and bandwidth (it's lots of real-time image transformations). So if you look at GAE it's just about the same as EC2 (0.10 / hr for CPU, 0.10/0.12 GB for data, 0.15/GB for storage), but there is no "reserved instances" discount which is really significant (unless I'm unaware of it). On balance it's comparable to EC2 but you have way less flexibility.