You don't say if you are buying or selling. If selling, its worth probably more to you (you have more invested) than what you can ask or get for it.
If you are buying, its worth is based on what you want it for, what you might have to invest in it after you find out what is likely wrong with it (truth is someone wants to get rid of it for a reason), and how much of a risk you are willing to take.
I looked for years for a JD 1010. It was the first 4 cyl JD came out with, and it was in the body of the time-tested MT, 40, 420, 430 series of the 50's. Good 3pt, good tranny, good workhorse of a utility size tractor (I had a 420 for years). I would probably figure one would be worth about $3500, give or take $1k for condition and desire to keep it running. I sold my 420 two-cyl for $5900 two years ago. The 1010 isn't the vintage material that the 2-cyl is, IMO.