Well I guess no one ever answered the question, and I also suppose it's really just an opinion. I find thst the most common answer to the "what is it worth" question is whatever someone is willing to pay for it. An inexperienced collector without much background knowledge who really wants one might pay a rediculous amount. As of 2016, the number of these machines appearing is dwindling rapidly, and most are now I believe in private collections/museums. The price of these machines will continue to rise over time, so really whatever you paid for it, youll eventually get your money back if it is an investment.
My valuation, and I should stress that it is my personal opinion, is that a working, clean museum quality blue badge pet is worth 1500-1800, and you might on a good day get away with 2500, but it would be way out of the ballpark. Like say someone who randomly saw it and went OOOO I want it right now! Trust and believe it wont sell fast at a higher price...but it will eventually. A working one with average wear and tear, 1000-1500. A non functional, but cosmetically very decent one, maybe 400-700. A "barn find" one that doesnt work, is filthy and will take a lot of work, 300-500.
Lastly one that is physically damaged(missing keys, doesnt work, severe rust, major dents, crt is trashed) depending on the extent of such damage should be priced as junk because only a very serious and skilled collector could bang the dents out, repair rust damage, spend oodles of spare time looking for chips, tracking down a suitable replacement crt\repairing damaged crt circuit. Some things are easier to find like replacement pet key stickers...but i repaired one with a broken 6520 uart. It took 3 months for me to find some. Memory is another issue. It is possible to make some "mod"ware and use a 2114 sram, but it requires some know how. 6550 memory is simply extinct.
So simply bare in mind these things when considering your purchase. If it just going to be eye candy? Buy a junk one and bang the dents out, fill and sand as needed, repaint. I myself however love vintage computing and a broken computer nags at my conscience every day until it is fixed and completely restored to new condition. Id never sell my PETs though, they are worth more than money.