They have an English price list, and you can find monthly on the bottom of the sheet.
I had a place for ¥33,000 monthly, only needed to pay ¥5,000 for change of locks, ¥3,500 matinance and ¥1,600 for Internet. Not wifi, so if you want it, you need ever a cable to connect your laptop or buy a wifi router.
Rihga Royal Hotel
2-14-2 asano north of kokura station, tel. +81-93-5311121, (http://www.rihga.com/fkokura/). the top hotel in town and the only rihga royal hotel in kyushu.