The “upgrade” moves your files over to the new look and feel, and doesn’t include new versions of software like most Mac upgrades. But, the price point and marketing set this up to be a big win for the company, particularly with online distribution being priced at that point.
Some argue compare this price to Mountain Lion, but I don’t think that’s fair given the substantial change to the OS. Mountain Lion has many changes but it is the front-facing UI that sells. The last time Apple made this kind of change to the OS was with Leopard, which I believe retailed for $129.99.