This is the company I used. Looks like basic service is about $150 for a month.
PuPuRu: Cell Phone Rentals Japan
if used only locally, then it shouldn't be too bad. You can pick up the airport, and they give you a prepaid return envelope, just drop it in the mail the day you leave (or have your host family do it)...it just has to be postmarked 3 days after the contract is up