This is already happening.I bought a used Radeon 7970 in December for $350 USD off ebay. Just yesterday I sold a New In Box R9 280x for $275.

This is happening for 2 reasons:

1> ASICS are already here
2> Nvidia's Maxwell architecture is better than Radeon in terms of cost vs performance

Nvidia 750ti cards are about $150 new and will run scrypt out of the box at 265KH on 35 watts! A modest over clock pushes it to 280KH and flashing the bios to boost wattage up to 65 will push the card over 300KH. I bought (4) such cards and put a pair into two separate systems. With modest software OC, I am over 6.5MH with Groeslt and a whopping 180MH with Fugue - from each 2 card system.

My buddy built a 4 card system and he is seeing just under 1.2 MH total for script @ 35 watts per card!

Radeon is dead for GPU crypto and doesn't even know it yet. Nvidia has stolen the crown because it is now an energy "sipper".