Amazon has a calculator for that purpose:
http://calculator.s3.amazonaws.com/index.html
Your EC2 base price for a dedicated M3.medium server will be relatively negligible compared to EBS pricing. Your EC2 pricing will be based on how many CPU cycles you use and your Outbound Traffic per GB. Even with an average moderate mail traffic of 80GB/mo In and 40GB/mo Out doesn't add up to more than $350/mo no matter how you slice it. Your EBS pricing will be around $3366.80 per month for two 16834GB volumes (which is the max available per volume), or around $1.6834 per user a month.
You can get further details about Amazon pricing at:
http://aws.amazon.com/ec2/pricing/
http://aws.amazon.com/ebs/pricing/