The other thing to consider is loss leaders. They may take a small loss on say a desktop computer, but they are counting on selling you a monitor, maybe you upgrade to a wireless mouse and back-lit wireless keyboard. They make money on those.

Sticking with the laptop example your going to want a cooling pad, a carrying case, an external hard drive to back it up. Even if you don't think you want those a good salesman will convince you that you do. You also need an extended warranty, maybe you want in home service for 3 years for an additional $500 (yes, oddly specific. I received a Dell laptop for my birthday with 3 year in-home service).