Something else to keep in mind (which was inspired by Chiyo's comments):
There's a wide range of bid levels for AdWords/AdSense. Some terms might fetch a bid of a dollar or higher, while others might earn a few pennies. It all depends on how much competition there is for each term.
Now, there may well be a point where the bid for any given keyword tops out, depending on what's being sold. Consider:
- A bid for a keyword related to power-plant construction might top out at $25 or $50 (because that price, high as it is, may be less than an advertiser is paying for leads from bingo cards in trade magazines).
- A bid for "Montpelier Ohio motels," on the other hand, might top out at a dime cents because (a) there aren'ta lot of motels in Montpelier, Ohio; (b) the profit on a Montpelier, Ohio hotel room isn't likely to be high; and (c) the traveler on Interstate 90 who books a motel room in Montpelier, Ohio may never have reason to visit that town (or that motel) again.
In other cases, bids may reach unsustainable levels because of competition but may fall as advertisers realize that they're losing money on every sale. But even if those bids fall, it doesn't follow that the bid for the power-plant construction keyphrase will erode--or that the already low bid for "Montpelier Ohio motels" will drop.
IMHO, it makes little sense to say that "bids will fall" or "revenues will drop," because bids are determined entirely by competition and other factors for each individual keyword. In some categories, unrealistically high bids may drop to more reasonable levels, but in other categories (especially categories that are just beginning to attract CPC advertisers) bids may climb until they reach their natural ceiling.