> The API servers that we are able to push this load with cost a mere $90/month

That would be about an EC2 m1.medium

It would be interesting to see how much it costs to run the whole cluster. I'd also love to know which part of the redis featureset they really use. Redis is great, but I think a lot of other database backends will give comparable performance when allowed to store all their data in RAM (MongoDB, Postgres, Riak, Cassandra, ...). The advantage of these (especially Riak/Cassandra) would be that for pure key/value semantics, they take care of all of the annoying operations overhead like rolling updates.