$50 - $100 is a fair estimate.
You can also check out ebay for wireless cards and occasionally you will get a very good price (like sub $40) or a low power card too.
Before you buy, do a research on power consumption of WiFi cards.
Some cards suck battery power like anything thereby reducing battery life on your Zaurus very fast while online, while some (like the Socket low power Wifi / Symbol Spectrum 24 Networker card) are much better at preservinbg the precious juice in the battery (though I dont think you really have to worry that much with a C3000, I had to cound every second on my C750 which has half the battery life of the C3000)
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I just got the SL-C3000, and am using an Ambicom wifi card with it - the battery life is absolutely stunning. Last night, I was using wifi almost constantly, for about 4.5 hours, downloading lots of ipkgs, surfing the web with Opera, etc. And still had about 50% battery capacity left after all that time.
I used to think that my Sony TH-55 had great battery life, when I could surf for 4 hours, before the battery tanked. ;-) Of course, the Clie isn't quite as much of a handful. ;-)