The cost of guinea pig food depends on exactly what you feed them - i.e. some dry mixes cost more than others, and veg prices vary wildly too. As a ballpark figure, two guinea pigs will cost you around four dollars a week - or somewhere between $2.50 and $5 - for dry food, and between one and two dollars more for fresh vegetables.
Guinea pigs love grass, which can add extra nutritional weight to their diets
In order to give them the widest possible diet and best nutrition, the fresh veg should be quite varied - broccoli and carrots are the best 'regulars', though. The carrots can be carrot tops discarded from your own kitchen preparations. Some fresh grass can be included in the diet too, and that may well come free, if you have access to a garden. Hay is a staple too.