In Las Vegas where virtually every casino has a buffet, the various publications which one finds in their rooms used to suggest $1, now they are suggesting $2. [Remember the waiters bring you a drink (orange juice in the morning, a soft drink in the evening) and coffee at the end of the meal--not a "big" amount of service--and periodically (maybe) will remove dirty plates from the tables.]
[Side note: In some areas of the USA one is FORBIDDEN from re-using plates at a buffet, each time you head out to get more goodies, you are supposed to use a fresh plate--so the plates can pile up if not removed regularly.]