I don't know if the earth ride is one of those specialty cymbals that may command more money always, or from time to time ,or at all,or if it is one of those that you can get for less because they are a drug on the market.
I guess you could figure value as what you would paid for a 20" A Zil,1979 or later,I think that was the first year for them. That would be 6.00 an inch,which is about what I would spend on a 20" A Zil 70's or after.
That being said I think the earth ride pre dates the paiste Rudes,and was intended as a rock cymbal,pretty much all ping no sing dry kind of ride,but soon after a lot of Jazz drummers started using the earth and rudes.
If you are buying it to play I think 120.00 is a good price,but no idea if you are buying it to flip.