I just spoke to apple store in Australia, and they are selling 10.5.6 for $158.
This gives you something to work with when looking at prices on Ebay, Amazon, etc....
To my astonishment, this is a lot cheaper than I could find on Ebay
Also, watch out that you don't buy the grey coloured discs as they are machine specific.
....and dtravis7, it sounds like the apple version of a candy store