Just my humble opinion as a casual observer.... (Not a collector or dealer and no interest in owning one)

It varies by condition. $50 for a garage sale '70s T&C model that's been sitting for 30 years and up. $100 for ones in rideable condition doesn't sound unreasonable. Once you start getting back into the '50's they become a bit more collectable and the pricing gets more subjective depending on condition and model. Frankly, anything under $250 for a nice, rideable old Schwinn Town & Country, Twinn, Stingray Twinn, etc... isn't necessarily an awful buy IF you have an interest in owning the bike. If you're looking to buy and sell to make money, just keep looking for those $50 - $100 models. With a little elbow grease you can usually double their value if you can find a buyer in the market for one.