Hmmmm, let's see...
If you don't plan on breeding the cat, you need to get it spayed or neutered. Spaying in females reduces the risk of a cancer (*forgot the name of the cancer but oh well*) and neutering a male tends to soften theiur attitude and they spray less. An un-neutered male will still spray occasionaly. If I'm correct, it costs 100$? $200?
Your cat should also get vaccines. Which costs money.
It's a good idea to get your cat microchipped (a small chip is implanted in the cat's skin so if it gets lost, it's scanned for the chip, which says your phone number, name, and address) or get a collar for it.
You also need to get it something to prevent fleas. We usually use a flea collar so you won't need to spend even more money on flea treatments.
If you have an outdoor cat, check for fleas and ticks regularly. Both can transmit diseases, and they just suuuuuck.
You also need to buy food. A lot of food. It's best to try out a few different brands to see which one your cat likes the most. Decide on what type you want to get: Dry food can be left out for several days, but wet food is usually more nutrient-rich and closer to your cat's natural diet.
Also, you need to have a steady supply of water. Cats prefer moving water. It won't hurt to buy a 25$ cat fountain~
A cat's food and water dishes should be cleaned regularly. Would you eat off of mold-covered plates?
OH, and very important: LOOK AT THE INGREDIENTS ON THE BACK OF YOUR CAT FOOD GODDAMMIT. If it has more plants than meats on it on the back, it's probably not good: cats are carnivores. They don't digest the plant material as well as meat.
You should brush your cat regularly to prevent hairballs and ultra-shedding. They can also take baths.
You should reeeeally clip your cat's claws every once in a while. Or else they will be all "MY CLAWS WILL MESS YO ASS UP, BEYOTCH".
Cats will kick up a boatload of litter if their box isn't the perfect size. You'll probably need to go through several.
There's a ton of different types of litters you can buy. I prefer clay litter, but if wood-based litter or paper litter is yer style, go for it.
If you're keeping the cat indoors, rescue your furniture by buying a good scratching post.
Buy plenty of toys if your cat will be indoors most of the day. Ping pong balls, paper bags, cardboard boxes, and strings work well. Always supervise your cat with a string, though - they can choke on it.
TL;DR: Cats take a carpload of money and responsibility.