Of course it's possible - look at all the stuff in the Mac Mini besides the G4 subsystem for $500 retail.
It's way more the volume than the R&D. I would bet that for companies making G4 PCI boards, which is a not a consumer market by any means, their cost of the bare parts alone is more than $300 in the kind of volumes they'd buy in.
It's amazing when you cross from the consumer market to non-consumer markets the price differentials, and it's not companies inflating their costs, it's just the economies of scale.
If you are buying volume, talk to the board makers you have found and I'll bet you find their price comes down significantly with volume. If you're not, your best bet would be to look for some kind of development board as they are often subsidized by the chip makers.
If you live with the latencies and inefficiences of Ethernet, you could buy the Mac Mini and ethernet it in, but my guess would be you'd spend more time running the data over Ethernet and the protocol stacks than you would number crunching.
Oh, and if you want some real number crunching, we sell TigerSHARC DSP PCI boards
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.