Can't remember exactly but from memory they were around the A$400-450 mark. Could be more now though that was a few years ago.
Doesn't seem cheap but when you consider the costs of four-linking: 2 coil springs, 10 rod ends, tube and other materials and welding and cutting consumables, not to mention time which I think people forget the value of (I know I do) they start to look pretty good value!
Maybe the four link is a better system for outright performance I don't know but a lot of quick cars still run leaf springs.