Not worth a whole lot. You can get them brand new, mate just got some wide 14" set for his Hillman Hunter from Melbourne.
Definately not the standard Mk1 GT rim, as they used a pressed stell rim with hubcaps and the Mk2 came with Rostyle Wheels. Very common aftermarket fit though, as the 5" wide 13" rim was pretty crummy on what was a pretty quick car, they also were used a lot in racing to keep brake temps down.
http://www.pixelmatic.com.au/cortina/gt.html has a bit of info