theres been a few threads like this before, the search button up there ^^ is pretty handy
but in short, yah $20 isn't bad...if your not a bit smoker than its alright, if you smoke a bit then buying q's, 1/2 or full ounces comes out a lot cheaper.
as for coke, it varies from city to city and varies a lot in quality....but anywhere between $180 and $240 in melbourne...
hope that helped