Grams

Within the United States, grams will cost no more than $20 (Unless it's extremely potent stuff which is horribly hard to grow) and no less than $10. In states such as Colorado and California, you may find a dispensary that sells grams between $7-$9, but these have low THC levels and are on the lower end of what dealers would typically give out.

If you're getting them illegally, three indicators of price are: Location, Strand, and Demand.

Depending on drug laws within your location and the tolerance of individuals of the state, the price may go up due to a higher risk for your dealer being sent to jail and facing harsh punishments.
Strands have subjective prices, but a good indicator is the levels of THC, which can be seen by the amount of orange color on the strands.
Finally, the demand of whatever strain will have a normal economic effect. If you want something common like Sour Diesel, it may be a bit cheaper, but Darth Vader OG isn't asked for often, so dealers and dispensaries can claim it costs more for whatever reason.