Docomo's Xi LTE Packet Houdai Lite will provide 3 GB at full speed for ¥4,985/month and be available from October, coinciding with the end of the Xi data discount campaign. After 3 GB, speeds will be throttled to 128 kbps, or additional bandwidth will be available at ¥2,625/2 GB. (Note 7 GB will cost considerably more with the lite plan.)
The standard price on the 7 GB flat rate Xi plan is ¥5,985. There is also a tiered plan, but it's stupid.
However, 3 GB could easily be enough for most users, depending of course on your streaming and tethering habits. Over the past 3 months, I've not exceeded 3 GB of Xi data. However, I am using a standard 3G phone (Nexus S), so I am not seeing increased speed and decreased latency of Xi. It is highly likely that I'd consume more data if I had a fatter, "4G" pipe to pump it through.
If you are currently an Xi user and interested in saving ¥1,000 each month, install a data counter (or activate the ICS counter if you are using android 4.0). As has been pointed out, the built-in counter doesn't have the ability to export the data to an SD card, unlike many third party counters. (Personally, I prefer 3G Watchdog.)