The rates are the same ($1 per million writes and $0.70 per million reads beyond the daily free threshold), but the daily free threshold is 0.05 million of each for master/slave, and 0.01 million of each for high-replication.

For small applications it costs more because of thresholds for free services is lower. In our case it costs a lot more since some of things we were doing need another instance on HRD that we didn't need on M/S