It really depends where you live and which stores you go to, the best thing to do it shop around. I have seen them ranging from $2-6, somewhere in that price range I assume would be about average.
Gerbil food is the same as hamster food. Look for hamster/gerbil food in your local pet store. The average price for food is between 2 and 7$. It also depends on what kind of food your looking to...
Typical costs:
Basic dry kibble, on average, costs between under $5 and $8 for a four-pound bag each month.
Premium or "holistic" dry kibble, on average, costs between $8 and $16 for a four-pound...