You pay per hour you use the server ; it depends on which instance you chose ; prices are around 0.15 cents for a medium instance
You pay per Gb around 0.12 - 0.15 USD cents ; first 10 Gb is free , per month
You pay for offline storage (EBS) , you need this else when your instance would be terminated you would lose all your data (database etc)
You pay for better storage speed (optimized EBS) and for optimized instances
Pricing depends on the physical location of your virtual instance (sic!) ; e.g. South America is possible but more expensive than locations in the US. Virginia is the cheapest.
Do understand that:
1. the power of an Amazon EC2 instance is in general not that fast compared to modern dedicated hosting
2. the price per hour adds up ; 0.15 USD per hour means 24 * 31 = 111 USD when you run your instance 24 hrs a day
3. the costs for traffic is not cheap ; 100 Gb means 12 USD extra
4. Amazon is not always 'that' stable , imho
Compare for instance with
http://www.100tb.com/ ... They offer 100 TERABYTE of data AND a beefy server for around 200 USD a month. Yes you have to handle a lot of things yourself, just like on Amazon.
An alternative is to use a cheap VPS. Those can be had from 5 USD (!) per month and could very well suit your needs.
Regards,
Walter