Weather I'm buying or selling, here is my take:
80%-100% tread = all you can get/give, probably within 75% of retail.
75% tread = 60% of retail
50% tread = 40% of retail
25% tread = 20% of retail
Less that 20% tread = $100/set or super cheap
Any tires now out of production should get dinged maybe 10% more since you probably can't replace them if you rip a sidewall etc.