Plus if you need a larger instance like a 7.5GB RAM instance, it's $800/month on Heroku vs $75/month for a reserved EC2 instance with comparable RAM. That's an astounding difference in price for cloud storage!

Of course, the real competition is other data-storage services like Amazon RDS, MongoHQ, MongoLab, etc. (all much cheaper)

(OK, so I excluded EBS costs from my EC2 comparison above but at $0.10/GB-month it's still going to be a huge difference for most cases)