I went there lasf month for a week. Everything is more expensive!
And like Az said, buy food and drinks! They'll treat you better.
And if you want have them weigh what you bought right in front of you.
Sometimes it's a custom. Try the "Sweet Dreams" coffe shop (the one with a big ass drawing of cheech and chong in front!)
or the Pink Floyd! Either way, i bet you'll have a great time.
But don't make it all about smoking, the city has a lot of other things to see! Like... well you'll see. Have fun!! :rasta: