Do you remember what your reasons were for moving to EC2 direct? I.e. needed how much more processing / memory / etc.? I know that when I start adding in some typical features (background processing, redis and / or memcached, wildcard domains, solr search, hourly crons) the heroku prices really start to add up. Significantly more than EC2.
For example the Heroku addon for 100MB of memcached is $20 / month... which far exceeds the cost on an EC2 box.