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.]

$0-$2 depending on the service. One can't be expected to tip the normal 15-20%, especially if you're basing that on a super inflated hotel buffet price (such as the $22.95 they're asking at the Doubletree I'm staying at now). There's no was I would pay $30 (after tip) for a shady buffet breakfast!