Well you start off only needing like 10-20 , and like most drugs your tollerance goes up, and i mean waay up, and you need 80's and shit just to get your "high". I thin they are fucken stupid and they ruin lives , i dont do them but my town is infested with them so i see them a lot.
Pretty much what capedankness said you need about 10-20mg. If you go for a whole 40 your first time, youll most likely end up puking.
.50$/mg (20mg : 10$. 40mg : 20$. 80mg : 40$). 5mg to buzz, 10mg for the high, 15mg to be fucked up, 20mg to be on your ass. Assuming you don't have a tolerance built up.