I've looked hard, but nowhere in the Heroku documentation can I see anywhere that actually tells you how much a particular dyno type costs in any other format than dyno/hours.
This makes sense since months are differing lengths and charging in simple 'dyno-hours' just simplifies everything.
That said, I've always thought it would be useful to have atleast a ball park figure for how much it costs to increase dynos for a short space of time. At Substrakt, we deal with a lot of clients who have prime-time TV ads and relatively small traffic the rest of the time. We could be running 2 Standard-2X dynos most of the time and need to ramp up to 3 Performance-L dynos for around 5-10 minutes a few times a month. Obviously, in this case the quoted $500/dyno/month price is of little value to us when we're warning clients of higher than average Heroku bills in the month to come. They see $500/dyno/month and, quite understandably, panic.
I threw together a quick spreadsheet just to remove the guesswork involved in this. It's nothing fancy, but will be very helpful for us at Substrakt and hopefully others!
These are best-estimates and not to be relied upon for accurate billing as it makes the assumption that every month has 740 hours (which is clearly not true!)