When is the rental for? I know there is a huge difference between seasons, if you are going in between summer and skiing seasons I'm sure you can get great prices. When I went I checked everywhere and I could get by a rental for 3-4000 yen less if I hand in the car at 7pm at CTS, which we tried last and it wasn't nice to be trapped at CTS (they don't allow people overnight at the airport - they force you to use the onsen). My usual nice rental place had no car at all when I was there.

It seems that nothing was totally opened yet when I was there in July. Even the melon buffet wasn't open. The onsen at CTS said that they couldn't serve proper breakfast because they had staff shortage (but they still charged the full entrance fee and nothing has been remodelled since 2019).

I was able to rent a K car at NRT Avis at 20k for 5 days in January 2020 (I didn't buy the full excess as my travel insurance covers that).