I'm a true believer in paying a little bit extra for a car if it's in exceptional condition. You save yourself alot of headaches and alot of money down the track.
So well looked after examples are worth spending extra on. You can still find parts cars relatively cheap simply because nobody wants them for anything but parts.
I think when they reach $30,000 they will sit there for a while. I have heard of alot of people spending $20k+ on restoring a 240z and alot of the time they are not what I would call perfect examples even still.
Certainly in the states some of the finest early cars have been on ebay for $18-$35k recently and as more people recognise what a true classic the S30 is the more people will desire them.
In Japan some of the performance models such as the 432 and 432R and 240zg's change hands for much more than that. I've seen the price in Yen on a genuine ZG and it translates to $35K AUD and it wasn't that flash.
One of the members on Classiczcars.com is keeping a record of all the VIN numbers that have been submitted to him of Z's in Australia forget his username right now but it will come to me and ill post it later.
For now you can submit information to Craig's www.wazcars.com
I should note I'm by no means an expert on any of this stuff and those who wish to provide objections or other opinions please by all means chime in here.