Is this perhaps a signal that Apple is approaching release of a new Mac Pro? The price drop is “highly irregular without something coming” (according to a well placed industry observer).
Get a 12-core 2.4 GHz Mac Pro for only $2499.99 at Amazon.com. The price over at Apple is $3799.99. Maybe other vendors have it similarly priced also, one can always look around. The 4-core 3.2 GHz Apple Mac Pro MD770LL is $2199.99, $300 less than Apple.
In short, Apple appears to be dumping models that might have too much stock, a move that makes sense only when something new is coming (or if we want to fear the worst, the elimination of a product line).
While you might have to wait a few weeks for the 13-core (as noted), saving $1300 is not bad and it’s via Amazon Prime, not some oddball 3rd party. The 2.4 GHz model makes a great video processing or audio workstation. The 3.2 GHz model is an excellent all-around workstation.