Good place to look, the Fairlane facts pages,
I saw a real nice ZF for $5000 which was at least $2000 too much, my ZH was $2600, and its quite a nice one, low km's etc.
has used car prices, for ZG, lets say 1975, it lists a custom (basic, with 302) as $700 - $1,600, and the 500 (better with 351) as $900 - $2,000.
That said, a good one now has a bit more pricewise due to its collectability, if you around Sydney and found a nice one for under $3000, I reakon go for it. Other places might have more or less available and more or less demand, look at the trading post for your area.
If you want a ZF (looks basically the same) message me and I can pass you on some details. If you want a ZH instead... look at the one I have for sale in the 'market place' part of the forum.
Good luck getting a couple of tonne of aussie made steel!