All of the points that everyone has made so far have been great and I totally agree, I'd like to note a couple things relating to food and vet bills that might help, as well.
1. For food, find a good quality dry and cans that your cats will eat, but won't break the bank...For me, Nutro Max is a pretty good dry and isn't that expensive, it is also available in most stores... For cans, I'm feeding authority (only available at petsmart). When looking for a cheaper, but good brand, email the companies and ask for coupons, samples, etc. That way you won't waste money trying things out. Nutro has a good customer rewards program and their products also go on sale regularly.
2. Once you've selected your brand, shop around. You can look at store circulars online and/or call stores to see if your products are on sale... Then, if you have any coupons (from the company,etc.) use those when the products are on sale to get the best possible price. I know this is time-consuming, but it can save you a lot of money (once I got a bag of nutro that was on sale for $10.99, I used a coupon for $2 off, so got a bag that was noramlly $14 for ~$8.
3. With regards to food, often, you can buy in bulk and save... I buy the 15 - 20lb bags of dry whenever I can and then store the excess in ziploc freezer bags in the freezer. one 15lb gag will last me ~3 months. You can also sometimes buy cases of cans at a discount. BUT I always take a calculator and check the price of a bag devided by the # of pounds in the bag to get the price per pound.. Sometimes store make you think that you are getting a good deal when the smaller bag is a better deal.
Yes, people do look at me funny when I take out the calculator in the catfood isle.
4. Vet costs- Although I have never used them, often local humane societies will give vaccinations at a reduced cost...like $5 for a rabies vacc. I would still reccommend that you go to your normal vet for a physical, but getting the vaccs at the shelter may say you some money... I know that it cost me $144 to get 3 vaccs and exams for my two kittens....If I had gotten the vaccs at the shelter, it would have cut off probably $30.
5. As for litterboxes, bowls, etc.... Sometimes you can find good pet bowls and litter boxes at the thrift store... I got a nice stainless steel bowl for 5 cents
Anything that you get used though, wash and sanatize thuroughly. Dollar stores often carry pet bowls, toys, etc. I would not, however, buy pet food from them (for the same reasons I don't buy human food from them).