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most likely it will cost the same as tiger, but we will not know for sure until prices are officially announced for leopard by Apple.
 
IF you recall the joke Jobs made, he said that it was $129 USD /£79 GBP for all the different copies of OSX (ie: all one of them).
 
The prices have been announced by Steve Jobs himself at this past WWDC:

Mac OS X Leopard...
  • Basic Version: $129
  • Premium Version: $129
  • Business Version: $129
  • Enterprise Version: $129
  • Ultimate Version: $129
For real. ;)
 
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