Well, cannabis is usually weighed out using the measuring system.

3.5grams is called an eighth. That means the 1/8, is 1/8ths of a full ounce of cannabis (ounce=28grams)

3.5g's varies in costs though. Some people can get shitty pot for $5 for 3.5g's, then there is quality cannabis that can be about $35-$40 for an 1/8.

Then, 3.5g's x 2=7g's. 7g's is called a quarter (1/4) which means a 1/4 of an ounce, or 25% of an ounce.

Then there is the next step, 7g's x 2=14g's, which a half ounce (14g's:1/2)

Then a full ounce (14g's x 2=28g's)

Costs however, vary from state to state, country to country, even cities apart there is big diff's, but here's a good idea of quality cannabis, not cheap, low quality junk, here in AB, Canada:

1/8=$35
1/4=$60
1/2=$100
Full ounce=$200
QP=$700
Full P=$1500

Now, there are dealers out there who sell by the gram, which usually costs anywhere from $10-$15, but there more cannabis you buy at once, the more money you save.

Peace