I have been on the most expensive overnight bus in Japan (called Dream Sleeper).
You can relax in a private room.
If you take this bus, you can save money on accommodation.

Fare:15,000JPY/130USD
Root:Tokyo→Osaka

【Related videos that I used as a reference】
・ Will Always Travel for Food – Riding the Overnight Bus Hotel in Japan

・LivingBobby – TRYING an OVERNIGHT BUS HOTEL in JAPAN!

・Kugaくが – 【神席】夜行バスの一番高い個室に泊まるとこうなります【ドリームスリーパー】|What’s the top class night bus in Japan like?(Osaka→Tokyo)

AloJapan.com