Have been on Heroku for about two years and can't recall a single price reduction in anything. I think it's pretty clear that pocketing the difference in the steady AWS price reductions is exactly what Heroku does.
We've dropped our prices a number of times, the lastest a few months ago:
We've plenty of customers who migrate to us when they "hit it big". It's no secret that that's where Engine Yard has been positioned for a long time.
The point of us dropping prices was so we could pass value onto our customers.