The value of a bike is only determined by what someone is willing to pay for it but this on-line guide from Hagerty's is a decent guide based upon previous sale prices,
I have the link opening onto the 77 Le Mans as an example. According to Hagerty's the Le Mans is valued much higher than a V7 Sport, I think this is based upon the eye candy effect of the resale red original colour scheme and all of the magazine coverage the Le Mans has had in the past couple years.https://www.hagerty.com/apps/valuationtools/1977-Moto_Guzzi-Lemans_I_850
It certainly does not list all the bikes of any one manufacturer but it does include a good cross section for a general guide. I am diving into the world of Laverda with the purchase of a 1973 750SF and the Hagerty site has been a good touch stone to the current value of the Laverda twins.