Working on a project for a client, I wanted to know what the most $-efficient way to keep stuff in ram on EC2-east. These boxes will only have traffic within the AZ, so there were no bandwidth costs.  I compared On-Demand prices with Reserved prices (assuming 1 year commitment for reserved.)

For High-Memory instances:
    about $21 per GB per month

  Reserved (1yr commitment):
    about  $14 per GB per month

non-high-mem instances are about 50% more expensive when looking at the cost as $ per GB.

Here's the google doc: