Monitoring AWS costs is tricker than you may think. Cloud cost management as a general topic is the bane of a lot of business expenses.
If you look at AWS Pricing page, you're going to have to get your head around a few things.
For example, your storage needs:
cool, sounds pretty straight-foward.
me: "I'll just have 2TB please."
not so fast.
which region do you want to keep your S3 buckets?
me: "oh, um, what's the difference?"
The main thing that comes to mind is 'latency' to where your users are being served. You generally don't want to serve content from the other side of the world. Also different regions can vary in AWS services available for that region if you need to use something specifically. Data sovereignty is also a consideration - are you allowed to store your users data outside your country?
But the main difference this blog post is worried about, is cost.