The exact details of the price cut are unknown, Dutch tech site Hardware.Info was the first one to notice it as pricing of lots of GeForce GTX 9xx cards in Europe suddenly dropped significantly. The site contacted one of the largest Dutch e-tailers and got to hear NVIDIA is doing a clearance promotion that runs from June 13 to July 31.
Here's an overview of the price cuts:
The price cut seems to be in effect in the US too, I just browsed Newegg and found several GeForce GTX 980 Ti cards for under $500. They even have a MSI GeForce GTX 980TI GAMING 6G GOLDEN EDITION for just $419.99 after a $30 rebate.GeForce GTX 980 Ti: $125 GeForce GTX 980: $75 GeForce GTX 970: $25