Firstly, well done for considering the shelter. It's so easy to just go out and buy a kitten, but there are lots of other cats needing homes - some tiny kittens, some older ones. If you go down the shelter route, they will ask you lots of questions about your home life, and will only match you with a cat/kitten that fits your needs.
With regards to food costs, dry food is your friend! Don't fall into the trap of giving wet food as a treat like I did, because they soon begin to expect it every night and costs mount up. Dry food is much better for their teeth and health in general, as well as being cheaper for you and easier to store. If you want to give a treat, a bit of tuna or even the inside of a fish finger goes down really well. Depending on the brand you want to feed, food costs will vary wildly. Ours will only eat whiskas so we buy a big bag from pets at home for about £10. It lasts us about a month, but it'd obviously last twice as long with just one cat. Be sure to measure their food out each day, as it's easy to overfeed. Don't feed cows milk as a lot of cats are lactose intolerant and it could upset their tummy. Plus once they get onto milk, it's hard to move them on to water. So just plain water is best from the beginning. If you want to feed some treats, PAH do tubes of freeze dried prawns, fish and chicken which ours love, and they last for ages. Whiskas also do some nice treats.
Insurance will vary depending on the age of the cat you buy and where you live, and the type of cover you require. I'm with tesco on their 'premier plan' and I pay £34 a month for two cats and two pedigree labs. I think for a young moggy you'll only pay about £7 a month, depending on the cover you need. You can choose your excess (the amount you pay towards a claim), and a higher excess means lower premiums.
You'll also need to get a worm and flea treatment regularly. If your cat doesn't go our, regularly could just mean twice a year (your vet will advise). If it does go out, you'll need to do it every month during summer, and once every three months over winter. If they catch mice, really you should worm every month, even in winter. We use a spot-on product called Advocat, as one application does fleas, ticks, mites and worms. I think we pay around £6 a time, and it's only available from the vet. But there are other treatments available from PAH etc which are equally as good and probably cheaper.
Most vets give you an initial check up free of charge. Your vaccinations will be around £40 (split over two visits) and neuturing is around £50, depending on your vets. I would also advise getting the cat microchipped, which my vets do at the same time as the vaccinations for £20. If you get a cat from a shelter, it should already be vaccinated and neutered. Even if you get a cat from a shelter, be sure to take it for a check up with the vet as soon as possible. You'll need to get booster vaccines every year, which will be around £40 again.
With regards to setting the cat up at home, don't go too mad. It's easy to get carried away buying beds and toys, but usually they just ignore them! You will definitely need a cat carrier, which you can pick up in PAH or online for under £20. Buy the biggest you can afford and it will last you the life of the cat - smaller ones might get outgrown. Once you've got your carrier, you don't need to bother buying a seperate bed - just put it in front of the radiator with some cosy blankets in it, take the door off and let the cat sleep in there. Not only does that save a bit of money, but it means the cat will come to see the carrier as its safe place, and this will save a lot of stress when you need to take it in the car. It also means that the cat carrier is readily available in an emergency instead of stuck in the back of a cupboard. You'll probably find the cat asleep on your bed most of the time anyway! Toy wise, those little balls with bells in and dangly things on sticks always go down really well and are very cheap to buy. The dangly things don't last very long though! Ours liked scrunched up bits of paper just as much as anything shop-bought.
For the litter tray, buy one with a hood to cut down on mess and smell (and give the cat some privacy), and put a cheap rubber bath mat in front of it to catch any stray litter. I find PAH really expensive for litter trays, but Argos do them cheaper. The best litter I've found is the Catsan Clumping stuff. It's like sand, and forms into a hard ball when it's wet. At first glance it seems much more expensive than the other cat litter, but it lasts so much longer. Anything wet forms into a ball, so you can just scoop it out, leaving clean stuff behind. And because it's like sand, it's also easier to scoop out the poo with minimum waste. My cats really seem to like the way it feels under their paws - they won't touch the gravel or paper type of litter. It also seems to cut down on the smell, as the grains completely cover the poo.
Hope this helps, sorry for any typos!