I was using Amazon windows instance for one of our client. We wanted to continue using this for some more time so we thought of exploring some options where we can minimize the cost (Reserved Instances).
Amazon offers Reserved Instance where in you have to pay much lesser cost for the instances that you plan to use for a longer period of time. The way it works is,
1) You decide how long you want to commit to the instance 1 Year or 3 years.
2) You decide the region in which you want the instance
3) Decide the type of instance you want to go for.
Be very cautious while choosing various options here because whatever you choose is not reversible or editable at all.
After this, once your payment is done, (probably at the end of the month when your bill is generated), you are eligible for the EC2 Reserved Instance, and your billing will now be at a very low price per hour. If you have a running instance it will be billed as a reserved instance or if you bring up new instance of the same configuration then it will be treated as the Reserved Instance. What ever option you go for the key is, same region and same configuration that you had chosen while specifying the configuration for Reserved Instance. If you have multiple instances of the same configuration in the same region then the one which is on for the longer period of time will be billed under Reserved Instance
(Amazon make sure that you pay less, they are being very nice to their customers :) )
How much do I save?
Although the usage charges are radically different for On Demand and Reserved Instances, there is a upfront cost that you pay for the later one. So unless you plan to use it for more than 5/8 months, you don't really save a lot. But if you have plan for using the instance for more time than this, then Reserved Instance is the best option to go for. For a longer use the savings are really significant.
I did some calculations for the instance that I was using Standard- Large (m1.large):
For 4 months you pay $1283 for On Demand Instance. And pay $1486 for Reserved Instance.
For 5 months you pay $1728 for On Demand instance, and $1630 for Reserved Instance.
So your saving starts from 5th month on wards.
For 6 months you pay $1468 for On Demand Instance, and $1428 for Reserved Instance.
For 7 months you pay $1713 for On Demand instance, and $1514 for Reserved Instance.
So your saving starts from 7th month on wards (obviously these figures are considering that your machine always on :) )
Savings would change depending upon the instance that you are using but surely you will save a lot in the long run.
Another interesting comparison is available @ http://spreadsheets.google.com/pub?key=rNtzGcAig_2X0xAP28V_2Jg