Note, I know this is an old question, I just want to counter balance some misconceptions about cost as I'm doing this right now as a test.
Unlike what DavidB thinks, it does not cost millions - even if you were to run dedicated hosted hardware, you'd easily be under a couple thousand/month (BTDT, one of my clients is running a 8 node cluster for about $800/month). That said, that's a maintenance headache you want to avoid, and Cassandra on EC2 is far easier to deal with.
You could easily run a substantial production cloud on EC2 for less than $1000/month and you can do R&D clouds for less than $100/month (I spend about $52 last month for an 10 machine test cluster). I highly recommend using TurnKey Linux to manage & provision your R&D farm, as their tools will allow you to migrate instances from your desktop to pretty much any virtualized hosting platform in a few minutes (and vice versa). Plus they have really slick integration with EC2.
For really serious levels of traffic, Pintrest once stated they spend $15 to $50/hour depending on server load, auto-scaling to meet traffic demands, see http://www.theregister.co.uk/2012/04/30/inside_pinterest_virtual_data_center/ for details
The real cost is in setup and manage of your distributed Cassandra instance. Luckily, NetFlix has just release a ton of manage tools just for this. You can find them here: https://github.com/netflix - there are also a ton of interesting videos about NetFlix's use of AWS, particularly moving stuff from Cassandra to S3 - see their blog here http://techblog.netflix.com/2012/12/videos-of-netflix-talks-at-aws-reinvent.html