CityVille has another way for players to buy a lot of items with CityCash and get rewarded for it. Shop And Win! requires you to buy a set value of items to win. What the prize is for reaching the full amount is unknown. The game mentions winning City Cash, Building Permits and 'more'. It's unclear if buying less than the maximum still gives you any of those rewards.
It's possible that the main reward is one of the 5 curvy apartment buildings - each being a different color. Those being the Pampered Palace, Majestic Mansion, Ritzy Residence, Deluxe Digs and Posh Pad. Limited Edition Sales give them as the special reward.