I find that casual eateries are some of the best places for solo diners as I often notice some people eating alone in them and the prices are quite reasonable. The ramen shops, tonkatsu, etc. in Tokyo serve some of the best Japanese food I've ever had at $5-$20 usd. Part of the charm of experiencing local life is eating where locals eat on a regular basis and those places are typically very affordable and < $20 usd for a meal. Me and my wife generally opt for a stopover in Tokyo every chance we get when traveling to Asia. I can't remember the last time we paid more than $20 per person for a meal. It's not common to find ramen priced over that in Tokyo. Nakiryu is very popular and ramen starts at 850 yen with their most expensive at 1400 yen. Add in an alcoholic beverage at 400-500 yen and you're still under 2000 yen.

I enjoy the nicer sushi places also from time to time but nothing beats a nice kaiten (conveyor belt sushi) as sometimes it's nice to try things that you wouldn't know to normally order.