In Northern California....
about $10 a gram
about $35 for 3.5 grams ($15-$45 depending on the quality actually)
about $50 for 7 grams ($40-$70 depending on quality)
about $90 for 14 grams ($50-$140 depending on quality)
about $180 for 28 grams ($100-$280 depending on quality)

The price is generally cheaper around October - November.

Although marijuana shops are abundant, it is real easy to just walk down a street and somebody will try to sell you weed.