I dont think the die shrink from 90nm -> 65nm will be reflected in the cost price of the 360. There's all the costs of the process of the new core, as well as getting Chartered Semiconductor ready to manufacture them too. It's not just IBM fabs producing the CPU, it's also licensed to Chartered Semiconductors to give MS a second source of them to meet demands... they already spent loads getting them up to scratch with 90nm with a huge engineering team from IBM going over there ... so a further hefty financial investment there if they too will fab the 65nm parts!
But that doesn't stop MS dropping the price anyway just to stay in the console lead!