Redbook is as about as unreliable as the Melbourne weather is at the moment, it might be used more by the insurance companies than any legitimate guide to a cars value especially a car that is 30+ years old. I worked in the car industry as Sales manager at a big dealership (many moons ago) and we used to laugh at Redbook back then!
There are a few 2+2's around some half reasonable but many are pretty untidy hence variety in price if you put that together with supply and demand you can get quite a variance. I've seen some tidy 2+2's go for as much as 10-13,000, these prices are for a clean (not concourse) pretty stock standard vehicle, for a 2+2 that is modified and Roadworthy and looks the part it won't be long before we see them reaching past the 15K mark, all this is only my opinion and observation though!
Hope that helps.
Rev.