I'm assuming you spent 20 dollars on some weed and want to know if the guy ripped you off. Honestly, there's no way we can really tell you, because it's all about the weight and how good the bud is. 20 dollars might get you a pretty fat sack of some schwag, or just enough to pack a bowl full of really good shit.

My suggestion? Smoke some of it, figure out how good it is, and decide if you're happy having spent $20 on the amount you were given. If you're not happy with it then don't deal with that guy anymore (or if you know him good enough, negotiate with him), if you're satisfied then great!