Japanese classes for low budget

Have you checked in Tokyo's Craigslist?

I used craigslist a few years ago to find cheap japanese classes, it may be useful for you.
Here's a list of Japanese classes in Tokyo: https://tokyo.craigslist.jp/search/lss?lang=en&cc=us&query=japanese

Also, check this out, do you know Duolingo? Its a free app to learn new languages and its super great.
You can learn Japanese in Duolingo now! look: http://www.japantimes.co.jp/news/2017/05/19/business/tech/gamified-language-app-duolingo-finally-adds-japanese/

"Duolingo is one of the most popular language applications on the market is Duolingo, a program that “gamifies” learning by rewarding players with points and new levels after they memorize vocabulary words and grammar points."
"Duolingo’s Japanese course is targeted at absolute beginners by teaching them all hiragana and katakana characters, and about 90 different kanji. The lessons will also focus on the grammar points and vocabulary words people will need to know to pass the Japanese Language Proficiency Test’s N5 exam, the most basic level."

Its only available for iOS right now but you can sign up to learn when it gets available for all platforms: https://www.duolingo.com/course/ja/en/Learn-Japanese-Online

Also, for volunteer japanese classes, check these links:
http://www.city.shinjuku.lg.jp/foreign/english/manabu/index_3.html
http://tokyo-foreigner.jsite.mhlw.go.jp/english/seekers_1/offer.html
http://u-biq.org/nihongo_tokyo.html

Good luck on your trip!