If you are trying to decide what kind of pet to get, it is very important to think about the costs that are associated with pet ownership. Pets will keep costing you money long after you bring them home. So its important to keep in mind costs like food, veterinarian visits etc.. and not just the cost of your initial pet purchase.
Here is a nifty little chart that shows some basic cost estimates. Of course these are just ballpark figures and can very a lot depending on the type of pet you get and also unforeseen circumstances (like illness / injury and costs associated with pet training or boarding)
CREDIT: The information for this chart comes from a post located here: http://www.petfinder.com/after-pet-adoption/estimated-yearly-costs-pet.html