Tethering isn't 'allowed' but it isn't blocked, at least, it worked for me a few months ago in Japan.

That said, I don't know why you'd want to subject yourself to tethering on a 128/256kbps connection...

International high-speed data passes are:

100 MB/1 day: $15
200 MB/1 week: $25
500 MB/2 weeks: $50

These prices are pulled from OLAM, I don't know if there's a public place to see them.