If you don't throw too much traffic at it you will be fine using it as a memcached alternative, but you will probably notice the overhead of an HTTP transaction. It largely depends on what you are doing.

It seems like you are referring to using it as a cache only, and on those terms, it really depends on your traffic profile. Chances are, though, with 200k uniques you would be pleased. If you are running in EC2, the bandwidth is free, so you only pay for computation. That can keep things pretty inexpensive.