Amazon seems to much overprice its services. If you look out for a similar size deployment elsewhere like linode or digital ocean(very competitive pricing), you'll notice huge differences. Ok, some services & features are extra but may we all don't need them necessarily & when you can host on non-dedicated virtual servers on Amazon you can also do it with similar configuration nodes elsewhere too.

IMO these huge costs associated with cassandra deployment are too heavy for small startups just starting out. I believe, If you consider a deployment for similar application using MySQL it should be quite cheaper/ affordable(though i'm not exactly sure). Atleast you don't usually create a cluster from the beginning. Probably we made a wrong decision to choose cassandra considering only its technological advantages.