I got mine and my wifes for free on letstalk.com and after rebates, we will both be getting a $100 check. New service only, so I ordered a new line and cancelled my old lines = 2 free 8320s with tmo. Great deal.
Click HERE to access older OS versions for 8700 and 8320 devices that I have saved
I looked for a couple weeks on eBay and new ones seem to be going around $300. Used ones in good condition around $250ish. I'd also suggest looking on Craigslist, you might be able to find a good deal there too.