Need tips/advice on inexpensive but private hotel accommodations for a solo Tokyo trip

Hey all! Taking my first trip to Japan in April and I couldn't be more excited. It has been on my bucket list for over a decade. I own a record store, so a lot of my trip will be record shopping. I need a private space because I'll be storing my records there across the 13 day trip before finally going and shipping them all home at the end.

I'm trying to find the most affordable lodging possible with the following criteria:

-Inexpensive (the dream would be $30-50 USD a night but that may not be feasible..?)

-Close enough to transit that will get me around Tokyo easily

-Private room with a private bathroom

-An onsen would be nice, but not totally necessary

I'm on Agoda, but I'm very overwhelmed and any advice or recommendations would be appreciated!