The unit cost of OS X is irrelevant.
Apple doesn't sell OS X for non-Apple-branded hardware, and Psystar doesn't sell copies of OS X per se. Psystar isn't taking any money out of Apple's pocket in a software market for 'copies of OS X', because no such market exists.
The place where Psystar is taking money out of Apple's pocket is in hardware. The difference between "an Apple-branded computer running OS X" and "a Psystar-branded computer running OS X" is the machine itself, not the OS. Therefore, the damage to Apple is the cost of equivalent hardware less the cost of one retail copy of OS X. Apple's retail computer prices are public knowledge, so adding up the damages would be trivial.
That's only where Psystar's barbed-wire enema begins, though. The big money will be in 'brand dilution'.
Apple's brand is immensely valuable, and Psystar has been trying to redefine it. Apple has always presented itself as a company that sells "the whole widget." They've spent a lot of money in every aspect of their business, from R&D to manufacturing, to sales and support, pushing the idea that a box from Apple is a single unit that Just Works (TM). Then along comes Psystar, telling everyone that Apple is really just a component vendor, whose OS division is just like Microsoft and whose hardware division is just like Dell. If you want an "Apple compatible computer," you can pick and choose pieces from any vendors you want.
A large part of this case revolves around whether Psystar has a legitimate right to tell the whole world what business Apple is in. At present, it looks like the answer to that is a big, fat "No."
If a court rules that Psystar has indeed been blowing smoke out its ass, then the equivalent advertising cost of every square inch of newsprint or second of airtime devoted to covering this case in the public view can be treated as damage to Apple. Every "they can't tell me what to do with the OS after I've bought the box" comment in these threads counts as noise that Apple will have to spend time, effort, and money arguing against. That time, effort, and money are resources Apple could otherwise have spent writing more software, designing new hardware, or otherwise expanding or improving its business. Psystar would have imposed an operating expense on Apple without having any legal right to do so. And following the theory that "if you light the fire, you can be held liable for whatever gets burnt," the court can rule that Psystar owes Apple the cost of cleaning up the mess it created.