Not so simple at all.

First off, there would be no "smoking wherever you want". If legalization were to occur, it would pretty much be legal to smoke in your home or a friend's home, and nowhere else. As it should be. You shouldn't have the right to blaze while driving, for example, or on a sidewalk where any kid could walk by.

Second, there's no way they would charge more than street prices if it were legal. If they did, they'd just lose business to street dealers. AND, the only reason marijuana is so expensive is because it's a scarce commodity. It takes time, effort and considerable risk to produce. If it were legal, lots of companies would make some bomb-ass shit on a large scale, and it wouldn't be so expensive. And if you didn't like the sound of that, you could always grow your own!

There are virtually no downsides to legalization (apart from more potential exposure to young children, like cigarette smoke, which would be a sad thing).