Never buy AppleCare at point-of-sale or from retail!!! There are many reputable sellers on eBay that will give you a significant discount. For example, I just got AppleCare for my MacBook Pro Retina 15" for $244 from a seller on eBay where it would cost $350 retail from Apple (and yes, the registration code provided worked).
Also, you have up to the year from the computer's purchase date to buy AppleCare. Don't be in a rush and shop for the best price.
I didn't even know that it was an option to buy it from 3rd party instead of Apple or some authorized dealers. If you can share the seller names of which you can confirm their AppleCare+ is legit, it would be appreciated.