Cloud Computing costs (AWS)

I’m trying to calculate what my possible costs could be with AWS. I want to implement EC2 very soon, and I understand we will need S3 for storage of our download content. I’m trying to understand what we will need, but I’m confused by things such as instances (I really don’t know what we need). I’m trying to assume that it could be a huge amount of data In/Out per month (with each download at approx 300mb, we could see up towards 9TB of outbound, and 1TB inbound data per month). Let’s assume that we will need a really huge system to cope with the demand ??" I need advice on what values I should enter into the cost calculator ( [login to view URL]), so I can understand the commercials. You should imagine that it will be as popular as an appstore, so really enormous traffic ??" this helps me to assume what our top costs could rise to. Thank you