The 360 price was 300-400
500 today is like 425 back then plus Kinect seems like a possible price.
Maybe they decided they won't take a loss on Hardware this time.
But I very much doubt an APU Made of a 7970/ and 8 Jaguar Cores. It would be an absolutely massive and huge chip. Expensive as hell and the Xbox would be bigger than the PS3 back then.