You will pay a one-time fee for a reserved instance, depending on the term.
Have you tried the aws calculator? http://calculator.s3.amazonaws.com/calc5.html
It should give you an idea of cost. But you will never be 100% correct as the values you input into the calculator are an estimation.
For example, a micro instance (linux), running 24/7 with a 10GB storage block will cost you around $20.00. A small instance bumps up the price to $60.00.
If you're using it only as a VPS service, you will find EC2 is more expensive.
aws becomes more attractive if you want to use the load balancing features along with redundant locations. But expect to pay for it.