You have to be a licensed developer of Xbox games to legally obtain one, and for that you have to send a portfolio to MS and be approved. More info on that at
http://dev.xbox.com.
I've also seen some people put them on eBay, but even if you got one on eBay you wouldn't be able to legally compile and sell games without being a licensed developer.
I'm not sure about the exact price, but what I've heard is around $20,000.
As for developing games without the SDK system itself, it's definitely possible, because Accession released the SDK programming files to the Net. The only problem is that you can't compile code using it without a license key which you have to obtain through MS. This could probably be cracked, however, so I'd say it's likely that it will be used for independent development. The modchip basically gives the consumer Xbox the only real benefit of the SDK, the ability to run unsigned code, so the only thing that needs to be figured out is how to compile this code, and I wouldn't be surprised if that's already been figured out.