Is it a good idea to study abroad senior year?

Yes, studying abroad will be expensive with any agency/organisation you go through. If I were you, I would check out AFS Intercultural Programmes. With Japan, you can go there for 8 weeks (I think that's what it is) and learn the Japanese language during the summer. You can go there for an entire year, stay with a host family and attend a Japanese school, or you can go for just a semester. The Prices range from about US$9,000 to about US$15,500. Or at least, I think that's the price range. However, if you think that the prices are WAY too much out of your price range, they offer scholarships to go to these countries. And, I think right now, they're offering 2 scholarships to Japan. But, yes, definitely check out AFS! All you have to do is go to www.afsusa.org and click on Study Abroad, and click on either Summer Pogrammes, Gap Year, or High School Programmes. Hope I helped!!!