AWS offers, pay as you go model, that is you only pay what you use.
Let’s take an example to understand this:
Suppose you are using say 10GB of space on AWS infrastructure, now usually what happens is, you estimate your usage say 40GB, reserve it, and pay for that 40GB monthly. But what if you are not using the whole 40GB. Like in our example, you just have 10GB of data, so if you are using AWS, you just pay for that 10GB, and you can always store more as your requirements grow, there is no restriction!
Have a look at this article for more details.