If you have a Discover card, they're currently offering 5% cash back on most purchases made through Apple Online. I was on the fence about upgrading my mid-2012 rMBP to a 15" rMBP w/ TB but the 5% cash back was incentive enough for me to pull the trigger. I sold my mid-2012 rMBP for $850 and am getting $130 in cash back from Discover, so I was able to get my new rMBP w/ TB for about $1,600.
As a bonus, if you don't have a Discover card, I imagine you can apply for one and still use the 5% offer if your starting credit limit is high enough to make the purchase, and they automatically double all cash back earned at the end of the first year. So if you were able to go that route, you could actually get 10% off after a year's time.
Just make sure you read the fine print if you do this -- you have to use their link to access the Apple Online store and the purchase has to be made online, you can't make the purchase inside of a brick and mortar Apple Store. It also takes up to 10 weeks to get the full amount back.