The best value I know of in Bangkok is the Japanese-run Kozo Sushi Bar's buffet in The Mall Bang Kapi (3rd floor) -- 200 Baht for an amazing quality Japanese fair, including sushi and various Japanese dainties. I myself very particular about Japanese food (ater 6 years dedicated to Japanese studies), but it's also every Japanese person who I saw eating there who instantly becomes a big fan of this place.

They also have a branch on Thaniya and a newly opened Siam Square branch, but I've never been to either. I've been to The Mall branch every week since last March (call me crazy but the food is SUPERB and only at mere 200 Baht).

My other fave is Khun Churn's vegetarian restaurant on Sathon Soi 10. It's mostly Northern Thai and the food, unlike in many other veg jonts dealing in TVP food, tastes wonderful. They serve only Thai herbal drinks and natural juices and the setting is in a little garden just off Sathon Road. The prices are more than modest, and they also have a lunch buffet on weekdays at 79 or so Baht (can't remember at the mo).