I haven't been to Vegas for a few years but I remember really liking the Caesar's buffet (think it was $20-30) and the sheer size of the Rio buffet was kind of neat.
I didn't really scour the strip for buffets when I was there. The food from the buffet at the Luxor was pretty good, but I couldn't speak to the value of it.
Best option is going for a late breakfast. Breakfast has the cheapest prices and if you go late you'll be there when switch over to lunch food which is usually the same as dinner.
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MOD EDIT: Removed for too much awesomeness.
You can get the "Eat All Day" passes at some hotels. Excalibur has it for $30, Luxor the same (I believe), and MGM Grand is $35-40 depending on the day. It's a pretty good deal.
Planet Hollywood was good last time I was there. It was somewhere in the $20-30 range.
Spice Market Buffet at Planet Hollywood is great. Best buffet in Vegas from what several people told me when I was there. Amazing value - it's like 7 buffets in one. And if you want "value" for your $25 just fill up on lobster and crab at the seafood buffet then top it off with a New York steak cooked to order at the American buffet.
My favourite buffet by far for dinner is the Bellagio... it's a little on the pricier side on the wknds at around $35, but is awesome! It's worth paying an extra $10 for quality.
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