Not sure where those estimated costs for running MongoDB on AWS came from. It jumps from a single t2.micro instance (which you can get for free) straight to 3x m3.2xlarge instances at $1500/month. That's a pretty big jump. There are at least 6 instance types between the two. Like 3x m3.medium instances with 500GB gp2 EBS volumes would cost $300/month. That's the on-demand pricing, you could probably save some more with reservations given the stateful nature of MongoDB.