That aren't guaranteed income. Apple aren't going to offer Macs at cost price or less on the off-chance that you'll buy enough accessories and software to make it worth their while. Computers aren't videogame consoles, which are useless unless you buy games. And even if that were the case then they wouldn't bundle software like iLife (which you should factor into your price).
Macs are expensive, don't get me wrong, but the iPhone is in a whole other league.
When the iPhone was released it was estimated that it cost Apple around
$250(£125) to make. It'll cost them even less now. They sell them for £279 and then make money over 18 months from the O2 contract. And given the ludicrous price of that contract I would say they're getting quite a bit
(some rumours say as much as 40%).
You do the math. Bottom line is that they're making back many times over what each iPhone costs to make.