One of the hardest things to get used to when starting with AWS is that you pay for almost everything that you use. One of the biggest benefits of AWS is that you only pay for what you use. This makes it hard to quickly answer the how much is it going to cost? question that often arises when people first start using AWS; they don't know exactly what they currently use!
In a traditional infrastructure or data center setup, many costs are paid during the initial outlay or in annual contracts. As AWS has no upfront fees, and few long-term commitments, so the usual thought process around costs is turned on its head.
There are a number of helpful tools to get a better estimate of your AWS usage costs. Don't forget that every AWS service page has a pricing section. While some pricing models can be a bit confusing at the start, it quickly makes sense.