Single-day pass: $15 for 100MB
7 day-pass: $25 for 200MB
14-day pass: $50 for 500MB
A better, but not very cheap solution is to just rent a wifi hotspot for around $8 - $10 / day if you're on a short trip. Get 1 gig / day as well.
I'm just going to get a japanese plan once I live here.
Single-day pass: $15 for 100MB
7 day-pass: $25 for 200MB
14-day pass: $50 for 500MB