According to CNet and Engadget, the upgrade for 802.11n support on Intel Macs will be $1.99 ($2 in my book). Not much, but rather sad considering you already purchased the hardware, this is merely a regulatory deal. I presume we’ll also see sales tax.
Now will Apple break the mold and deliver 802.11n upgrades for older hardware? I’d love to eventually upgrade my Mini, but don’t really like the idea of an ugly external adapter hanging around my desk. Internal is so much cleaner. If I do have to go that way, I’d likely buy an Ethernet bridge rather than any sort of USB adapter, since that doesn’t waste USB bandwidth and won’t hog a USB port.