I agree with Boxxboys values statement, they are all over the place since in the Corvette world price depends on heavily on two words, "origional" and "Survivor".
I am a former NCRS member and previously owned both a '68 and a '69, one roadster and one coupe, one a 4-speed and one an auto, both were of the 427/390 HP variety.
I have an old Corvette website with some info on it for all years up to the early 90's called The Corvette Place. Just select the desired year from the drop down menu. Feel free to use the casting number and production number information provided to determine how intact the car may be.
For what it's worth, at 20K I do not think you can go wrong.
PANTERA INTERNATIONAL
A DE TOMASO CAR CLUB
Presents the De Tomaso Forums
The On-Line Meeting Place for De Tomaso Owners and Enthusiasts From Around the World
Clicking on the banner will take you to the sponsor's website.