Before going to Japan for me to exchange US currency to Yen at SFO, the fee charged is almost 11%.
Although, in doing research Narita seemed to be a popular airport with a great rate on currency exchange, in my case, that is not the airport I am flying into (well, not me, I don't know how to fly, but the plane does).
To be honest, I have had difficulty in trying to find this information on the internet. I couldn't even find anything on Haneda's web site, so if there is anyone who has this information it would be greatly appreciated because if the rate at Haneda isn't much better than at SFO I'd just as soon get the currency exchange taken care of before the long flight.
If it's not too late, let me give you some info. It is always a lot better if you exchange in Japan (anywhere in Japan, Airport, Hotel, or post office). In addition, you can use credit cards to get the best rate (most place (or case, including taxi) takes credit card). There is a money exchange booth after passing the customs. You will get ~98-100 Yen per 1 USD (no fee). Hope this helps.