Gohante is a super cheap, no-frills Japanese restaurant that feels like the kind of place you could really get familiar with. The atmosphere is a lot nicer and it's slightly cheaper than Botan, though it gets really busy in the evenings. There's a lot on the menu, including sushi, ramen, grilled fish sets and everything else you'd expect from a Japanese restaurant, but it can be difficult to delve into more obscure stuff on it if you can't read Thai or aren't familiar with Japanese food. There are plenty of offerings with pictures, though.
A simple meal will cost you 50thb per person, but if you want to go all out on the sushi it probably won't go over 100thb.