Yeah it does cost a bunch to get your licence from scratch in Japan, most people go to the 30 day "guaranteed pass" schools I believe, but I am not 100% sure.
As a foreigner, having a driving license already lessens the costs significantly, as stated an IDP will last for a year, if you renew if after that you are taking a gamble. The Police are aware of the law surrounding the 1 year usage limit of international licenses and should you get pulled up you can get a hefty fine, worse yet, if you get in an accident your insurance company are able to weasel their way out of covering you by contesting the validity of your license (should they have the smarts to do so - you can always get lucky). Also be aware your IDP is invalid without your own countires license to accompany it, should you get in an accident you will also want either your passort or gaijin card.
I believe Americans do take a cut down version of the practical and written test upon production of a valid US license, but it is not the same for everyone. As a UK citizen, I got off light, all I had to do was get JAF to translate my license and then go to the Traffic center （交通センター）a couple of times and suffer Japanese bureaucracy and an unflattering photo coupled with some rather over priced stamps and I got a 3 year licence （初心者）. All they bothered to do was test my eyesight and ask me if my hands moved. They have to make a million copies of everything you give them as well and you will have to fill out a form to say you haven't commited any crimes or caused any accidents etc. To be honest my Japanese wasn't great when I took it so I was slightly confused by a couple of the questions but "no" seemed to be the route to go down.
As always YMMV. I get the impression the guys I dealt with were worn out desk drones who just wanted the whole affair to be over with as quickly as possible
The process is called "Gaimen Kirikae", and someone at the traffic center should know about it, most of the information was in Japanese only when I went and there were no English speakers, but to be honest you can probably just slog your way through it with very basic Japanese.
Here are a couple of handy sites about it, as always there's a wealth of information on the web about this sort of thing
In a nutshell, my advice: When you land, start finding out where to go to do all the stuff you need to do. It may be a pain in the ass, but much less so than taking the test from scratch or the potential fine and loss of job if you get caught stretching an IDP.