Question: I am hosting my application on amazon ec2 , on one of their micro linux instances. 

It costs (apart from other costs) $0.11 per 1 million I/O requests . I was wondering how much I/O requests does it take when I have say 1000 users using it for say 1 hours per day for 1 month ?

I guess my main concern is : if a hacker keeps hitting my login page (simple html) , will it increase the I/O request count ? I guess yes, as every time the server needs to do something to server that page.

Answer: 
There are a lot of factors that will impact your IO requests, as @datasage says, try it and see how it behaves under your scenario. Micro Linux instances are incredible cheap to begin with, but if you are really concerned, setup a billing alert that will notify you when your usage passes a pre-determined threshold - if it suddenly spikes up, you can take some action to shut it down if that is what you want.

<https://portal.aws.amazon.com/gp/aws/developer/account?ie=UTF8&action=billing-alerts>
@Dilpa yes you are right. If some brute force attack will occur to your website eg: somebody hitting to your loginn page then it will increase the server I/O if you enable loging for your webserver. Webserver will keep log to it's log files of every event and that will increase your I/O. Just verify your webserver log for such kind of attack and you can prevent them.