What they do is take the whole system apart and see what its made with and how much it actually costs to make.
Here is one, a company did for 360 (I havent seen a Ps3 one yet)
http://arstechnica.com/news.ars/post/20051123-5621.htmlQUOTE
First up, market research firm iSuppli, best known for taking apart consumer electronics to determine how much they cost to manufacture, has disassembled an Xbox 360. The verdict? Microsoft is taking a bath on the hardware, to the tune of US$126 for each high-end Xbox 360 sold. The ATI-made graphics processing unit alone is estimated to cost US$141 (including the RAM) while the IBM Xenon CPU accounts for another US$106.