Gee those RH's are pretty...
A good rule of thumb for a guesstimate of the cost of items is that money usually halves in value every 10 years. Or put another way, you'd need twice as much money in 2018 to buy what you can today. So, a $1 item in 1978 requires $2 in 88, $4 in 98, and $8 in 08.
To see how accurate that rule is, an XT500 was around $1100 in 1979 if I remember right, so that means a 500cc trailbike now should cost something like $9000, which isn't too far off the mark.
The $650 RH would therefore be selling for something like $5-6000 at that price.