A Mac's price has never really been dependent on the cost of the OS. Expect Macs with Leopard to cost the same until the hardware itself is upgraded next time. Then it's most likely that you'll get better features for the same price. Every once and a while, however, they lower the price a bit -- by $50 to $100.
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But, as CosmoNut has said, the prices will most likely remain the same.