Direct customers (e.g. not the Mac App Store) can upgrade for $75. Recent purchasers (via the direct channel and after April 10, 2012) can get it for free. In addition, every one can get it for $50 (half-off) for the first 24 hours, which is a great tactic if you want to elicit impulse buys through a channel that doesn't support refunds—I'm looking at you Mac App Store.