t2.micro is 1cpu/1gb RAM although cpu performance is burstable so you collect idle credits when the cpu is idle and you can use them when the cpu needs more than baseline, which is perfect for a dev server thats idle a lot. Performance on my Dev server is excellent, on a Linux box with no gui and not a lot running other than eqemu / mysql I rarely see usage over .25 even with 4-5 users on. Internet bandwidth is obviously amazing.
Obviously when I go to a more widespread beta I'll have to change the instance to a t2.medium which increases the cost. But you can make that change quickly via the AWS console (needs an outage) so you can afford to be optimistic. Moving the database server to another box is as easy as cloning the instance and turning off the eqemu server on the db box and the db on the eqemu box and reconfiguring.
As your server grows you can jack the instance resources up to match, and hopefully as the cost increases player donations increase as well. You can lock in a certain level of performance and get a large discount by prepaying fully or partially as well once utilization stabilizes.
To me it would take a long ass time to rack enough enough usage charges to equal a couple of decent servers sized to max possible capacity requirements, not to mention I don't have to pay my ISP for enhanced upload bandwidth (which would be $40/mo probably) or host a physical box somewhere. Last time I hosted a decent sized physical web server and database server at a data centre it was $400/mo each for rent on the boxes and the services, although that was for a short duration project (100hoursofastronomy.org) so no discounts.
More info at
http://aws.amazon.com/ec2/instance-types/
You can get a t2.micro (Linux) instance for 1 year for free as well if you're a new customer.
Regards,
Mg