bait and switch

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A fraudulent advertising claim or sales offer for a product or service that is not available or not for sale at the stated price, made with the intention of luring customers who are then offered something else or forced to spend more money to get the same thing.
• noun A deception based on a false claim or enticement that proves to be disappointing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An unscrupulous and sometimes illegal sales technique, in which an inexpensive product is advertised to attract prospective customers who are then told by sales personnel that the inexpensive product is unavailable or of poor quality and are instead urged to buy a more expensive product.
• noun by extension Any similar deceptive behavior, especially in politics and romantic relationships.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a deceptive way of selling that involves advertising a product at a very low price in order to attract customers who are then persuaded to switch to a more expensive product