The worth of a used Jeep (or anything at that) is purely being able to find someone willing to pay for it. I have found over my years of being Jeep obsessed I can look at a Jeep for sale and somehow in my head figure if the asking price listed is something I would be willing to do.
I think good advice for someone selling a used Jeep is look at your Jeep from the buyers perspective. Figure out all that has been put into it except for your labor and be realistic. Labor is usually not worth much if anything. I probably spent years working on a Jeep if I added it all up. If you have kept receipts for all your parts (usually something shocking if you add them all up) make them available to the potential buyer.
My best advice about figuring out what your Jeep is worth. Look at eBay, find other used Jeeps, surf the net, look for Jeep related forums and see what others are selling and how much they are selling it for.
I read somewhere that old Jeeps are rarely rare, collectible, or otherwise…I don’t believe it. I can justify any Jeep purchase easily. “It needed to be rescued.”