Gerbils are fairly inexpensive pets to care for. The initial costs of getting the gerbils and setting up their home will be the most expensive. ($125+)
20 Gallon Aquarium $40
20 Gallon Screen Cover $15
Water Bottle and Guard $7
Chinchilla Sand $5
Some things like an aquarium can be purchased from Petco or Pet Supplies Plus for significantly less money during their $1/gallon sale. These happen 3 or 4 times a year. Aquariums can also be purchased second hand or even found for free. Many retailers offer online subscription or repeat delivery services, and these can often be much cheaper than the in-store price.
Each month you will need to buy more bedding, food, and hay. Toys and treats are also fun to get for your gerbils and certainly enrich their lives. A monthly budget of $20-$30 should be adequate. In truth, a bag of bedding, food, and hay may actually last much longer than a month, depending on what size bags you buy.
It is unlikely that your gerbils will need medical care, but as gerbils age they may need medical attention. Veterinarian care can range from $40 to $100 or more. You should definitely research vets around you as the day you need one you will not want to spend your time looking for one.