I suppose it depends on the materials you use for the cage, how often they need to be replaced, how elaborate you make the cage, and what quantities of bedding, hay, pellets, and vegetables you use. Then again there's the expense of neutering (albeit this is a one-time thing), how often you bring them to the vets and how much the vet charges, if you need to leave the heating on to ensure they don't get cold, petrol used to transport the pigs around, and other such expenses.
Then again, they're expenses not unique to Guinea Pigs.
I don't know how much my
cavies will pan out each month as I haven't had the for long.