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The current price for a G4 Pegasos, valid as of 4:55pm EST:
This is due to the strength of the Euro and weakness of the Dollar.
The cost in Euros is 499 for the G4 model, so you can keep an up-to-date accounting of this.
As for Mac OS X, one can run it or a selection of other OS's on the Pegasos using Mac On Linux.