Just to expand on this, if you spend $2-5 million on reserved instances, you get a standard 20% discount[1]. If you spend more than $5 million, Amazon just say "Contact Us". Heroku certainly qualify for the "Contact Us" discount: at an average of $1,000 per reserved instance, they only need 5,000 servers, yet they're hosting well over 1 million sites.

Heroku provide a service that significantly simplifies the deploy experience, though you pay a premium for that privilege. If you're willing to take the complexity hit of deploying to EC2 instead of using Heroku due to price, you're not really Heroku's target market. I'd expect a price decrease eventually, though likely in reaction to direct competitors rather than the drop in cost of the EC2 infrastructure itself.

[1]: http://aws.amazon.com/ec2/pricing/#reserved-volume-discounts