Another factor affecting the price of ALL vehicles are government regulations. In 1970, how many bikes had catalytic converters? How many met emission standards? (The H1 was a good bug sprayer).
My 1971 SL100 didn't need a green sticker.
My 1971 R60/5 has a 7 digit VIN.
The times, they are a changing.
Now, to sell a bike, you have to meet noise regulations, emission regulation, safety regulations, NHTSA safety recalls, etc. All add to the price of our bikes. I have a 15 digit VIN on my XR250!
I remember in 1978 a new kz650 was $1700 if you bought the custom with the mag wheels. In 1979, emissions standards tightened, Kawasaki added reed valves on the exhaust ports to suck in clear air to reduce hydrocarbon emissions. That added $300 to the price of the bike. That was almost a 20% jump right there. (Of course, with the Carter administration, inflation was probably 10 or 12%, so it wasn't all emissions.)
More regulation means more cost. Just a fact of life.