The value of such a bike is bounded _above_ by higher models in good condition that you see on e.g., Ebay or http://www.craigslist.com. Right now there is a very beautiful 1974 International for sale for $449.00, and a professional that is bid at $501.00 or somesuch. An older classier 1971 International right now on craigslist has not sold for $600.00. A month ago a professional ridden "4 times" sold for about $900.00 (it really did look like it was ridden about zero times.)
So a lot depends upon the condition of the components.
I have watched ebay for the past 2 years and i really wanted either a 1972 Gran Sport or International (gran sport in 1972 was $225.00(?), international was $325.00). I have seen gran sports fetch between $200 and $300 for ones in very good condition (e.g. decals intact). There was a quasi-NOS 1974-6 competition (almost the same bike) that went for $300 or $400 a month ago but with much more valuable drivetrain and now i think it's back on ebay, currently drawing $150.00.
On the other hand there was a New-In-Box 1979 Supercourse that went for $800 just a week ago, what a surprise !! One problem with the Gran sport is that the drivetrain has almost no resale value, the simplex gears are junk - only the crank has value. Other bikes with campy or high end simplex derailleurs and stronglight cranks have a drivetrain that could go for $200.00 as parts.
In the advanced search section of Ebay you can search for sales of completed items. There are currently no grand sports listed but by searching similar models (e.g. Competition, 1969, $301.00) you can get a rough estimate of resale value.
Don Gillies
San Diego, CA