20 dollars worth of marijuana. depends largely on the quality. very low quality mexican schwag can be 3 grams for 20 dollars. high quality stuff on the street is generally 20 dollars per gram. at 275 dollars an ounce, which is about standard, it comes out to a little under 10 dollars a gram. Most dealers are willing to sell a gram for 15 dollars, or about a gram and a half for 20. It depends a lot on the dealer. Many street dealers here in berkeley will actually sell 3/4 of a gram for 20 dollars (see "custie").
i have also heard the phrase "dub for 15", meaning selling what would usually be 20 dollars of marijuana for 15 dollars.