There's no real guidelines on how expensive a cat is going to be. Some cats truly require vet visits, operations, and expensive dental cleanings. Most cats, however, get a yearly vet visit and that's it, at least for the first several years of their life. To be honest, my 15 year old cat has only been to the vet to be neutered, he was a feral kitten, and healthy, and I honestly think the trip to the vet would be more dangerous to him than anything the vet might've found. His level of freaking out is extreme, though. So, considering you take the kitties to the vet once a year, and a normal check-up is about 50 dollars as long as nothing's wrong, that's roughly 100 dollars a year. If there is something wrong though, things can get expensive very quickly, it is best to prepare for this in case you're faced with the option of a 1,000 dollar or more vet bill or having your kitty put to sleep.
For litter, my family buys cheap stuff. We don't really think it matters. Although I'm sure people swear by things like World's Best. We get ours at Costco; 50 pounds of litter for 10 dollars. My cat doesn't seem to have any ill effects using it. The general rule is to scoop at least twice a week, every day being best, and to change it at least once a month. There's also the general rule of having a litter box for each cat to prevent territorial disputes, but you may not need this considering they're both going to be kittens. I'd still say buy two litter boxes and have them in separate rooms just to be on the safe side.
For food, it's best to feed raw, and barring that, as many people don't want to do this/have no time for it, feeding wet is best. Higher quality wet food would run you 70 dollars a month, if not more, at least that's the case where I live. I'm also taking into consideration that you'd buy the bigger cans, as the smaller cans don't get you as far for the price. If you were to feed cheaper wet food, it would be more like 30 dollars a month, but the benefits of better food pays off. If you were to feed kibble, or free feed kibble on the side of the wet food (I do this) it would cost a bit less because kibble is cheaper and not as good.
Cat toys: Paper crumpled into a ball, a stick with a string on the end... they work wonders in place of the more expensive toys. But a pack of little mice toys doesn't cost too much, either. Scratching posts are important too unless you want your rugs/furniture destroyed... they can be pricey, but it's worth it. Or, you can make your own if you have the right skills and the right tools.