I'm not convinced the 880 will unseat the 780ti at 1440P/1600P/ 4K because of the lower memory bandwidth.
Even if the 780/780ti get discontinued, you will still have Titan Black at the top available until Titan II makes its debut, which means I doubt you will see 780ti drop below $599, and I doubt 780 will drop below $399, and there are already clearance rebates starting to flood the online retailers.
Just this past week, Ive seen MSI, Asus, Zotac, & Gigabyte 780ti's all in the 599-609 range, and the stock emptied out pretty quick.
Depending on the MFR $599 might be the time to buy a 780ti.