About the price range- yes, definitely low price, same like Polaris10 desktop card,AMD with this product is aiming for market share in all devices. It should be named M490 for mobile parts- don't buy R9 M485X, that is old, not power efficient part. There is a smaller new Polaris11-M480 chip as well, but it is meant for 1080p slim laptops running games at mid-high; Polaris10 is about twice as powerful. Regarding release dates- it could be late July, but I'd wait and see if there is an official announcement on laptops on 29th of June.
As for the GP106 from nVidia- release of these chips is currently expected in September, and speed- very similar to Polaris10.