I think $50 under the Apple Refurbished price is a fair guide, give or take maybe $20 in either direction.
I purchased a used ipad 16gb wifi for $325 that had a screen protector on it from day one and no marks anywhere. It also included the apple case and a penguins case, which I was able to sell for the $25 difference. I also didn't have to buy the $22 Zagg screen protector as anticipated.
IMO, if you want the newest iPad, go ahead and sell your current one on Craigslist. At least in Pittsburgh they seem to have been moving quickly. I anticipate keeping mine for 3 years at a cost of $7/mo (assuming i can get $50 for a mint condition iPad 1 four years after release).
Given the current price trends, if you really want to always have the newest iPad you're going to be paying about $16/mo. All things considered, that's not terribly much more for what you're getting. Just do the responsible thing and make sure your old iPad makes it into the hands of someone else that wants it, one way or another.
Also, always use protection. It only takes one tiny mistake for you to end up with a scratch on your screen. Keeping these products in good condition minimizes the environmental impact of these devices (by keeping them desirable longer) and maximizes your resale value.
Thanks,
Ken