I've heard that monthly mansions might be a good option to regular
apaato/manshon in case of relatively short stay (less than 1 year). But
it looks like in Fukuoka they're way more expensive than a regular
apaato:

http://www.weeklymansion-navi.com/fukuoka/
http://www.monthlyfukuoka.com/wls_ohorikita/

For instance, the rent for a certain 1K is 90k per month (roughly
double the regular rent). Others are more expensive. Looking at the
pictures they do like newer, better and more furnished than the regular
apartment. They even have elevators. Still, the difference is quite
big. What am I missing?

The first link carries a comparison with business hotel, so maybe this
kind of monthly mansion is intendend for really short stay (1-2
months)? 

Rafael Caetano