I bought an unlocked Motorola V3m Razr quad band for 50.00. I go to Ja 4 times a year so I keep it active to retain my phone number and the phone itself is bulletproof. Top up every so often and give the credit to one of the kids from the handset here in the States.
If you're going to be in Ja. often and want a permanent phone number that's the way to go. If not any cheap phone will do