Considering that you can buy a brand-new Kindle with a ton more features than the original 2007 model for $70 direct from Amazon...your answer is "not very much".
A quick search on eBay tells me you're lucky to get $40-$50 for it.
A major selling point of the original Kindle is the SD card expansion and the user replaceable battery. Unfortunately, the trade off of the lower quality e-Ink display, 3G wireless only on the Sprint network, and lack of native PDF support, X-Ray and all of the newest Kindle features kind of make it a harder sell.
I owned the original back in the day and have had the K2, K3, and Paperwhite 2012. Considering that the original was $400 and you can buy the latest tech for as low as $70, pretty soon, Amazon will have to pay you to get one.
