This is my motorbike
This pic is at least 15 years old, this bike does not any longer have the panniers, the faring and it now has a 4 into 1 pipes as those chrome ones rusted out. Ever since my seperation its been unregestered as I cannot afford to keep so many vehicles on the road at once (I have 3) with my present finances. Its sitting in my shed and one day I'll put it back on the road. Its 26 years old and it only about 35,000kms (21,000 miles) on the clock.
In answer to your question, as Lisa said its supply and demand, the value of any vehicle depends on its condition, mileage, popularity of the vehicle, replacement cost of the new current model and demand. I feel if a person is selling a used vehicle for close to what they paid for it new, it will be hard to sell as usually most new vehicles do depreciate.