They're worth what you can get someone to pay for it.
Worth implies value, which is subjective (Read Zen and the Art of Motorcycle Maintenance).
The POS is worth nothing to me, BUT you have to consider; How many 1981 monkey bikes, in good shape, are left out there? I'm gonna bet not many.
I own a 1974 Yamaha production racer that is, for all intents and purposes, brand new, zero miles.
As a rider, the thing is worthless. It is dated, hard to get parts for, non competitive in vintage race classes... and who in their right mind would RIDE a brand new 50 year old bike?
Somebody is going to give me box full of monies for that thing... cuz it's the only one left.
In the world of collecting, scarcity = value.
... if you can find a 1969 monkey bike, with suspension, chrome fenders and lights (all OEM), in decent shape, I have a buyer that will give you upwards of $10,000USD for it.
... any Ducati bevel twin sells for STUPID money, where a Laverda twin of the same vintage, which is a superior motorcycle in EVERY way, sells for a third as much.