You will probably find out your departure date a month before you have to leave. That gives you about 2 weeks to find a discounted ticket.
Megumi,
Shame on you for hijacking this thread! Why didn't you start one of your own? Spouse visa stamp will cost 4000 yen. If you want to leave the country at any time, you will also need to buy a reentry permit, which you can get same day. Single reentry permit costs 4000 yen; multiple reentry permit costs 6000 yen. Both are good until the end of your visa.