First off let me say that Chihuahuas are a wonderful breed, I couldn't imagine living without them, but you must make sure you are committed and take on the responsibility because they will be with you for many years to come.
They cost around £500-£1200 as far as I am aware in the UK
If you are buying one, make sure it is from a reputable breeder and do your research beforehand(they can be very hard to adopt in the uk)
You will need money for the initial vaccinations, microchipping, spaying/neutering if you choose to do so. The prices vary depending on your vets.
At my vets vaccinations are around £50 for the course and microchipping is £25
You will also need to invest in a leash and a harness (I prefer harnesses to collars for chihuahuas)
Other things are brushes (esp if you are getting a long coat), food bowl, water bowl and possibly a comfy jacket for the cold winters
Depending on where you buy them from and what you buy you can spend upwards of £15 for this.
You need to get some sort of dog bed too for your wee one

You must have enough money to take your chi to the vet, if God forbid an emergency happens and so I would recommend insurance. All of mine are have the kennel club insurance, but you can shop around. For me they work out around £12 per month per dog.
As for food, I feed mine dry kibble because I personally think that wet food can ruin their teeth. I spend around £40-£50 per month on food, but then again I have 3 mouths to feed

Chihuahuas don't generally eat much.
You might want to invest in a playpen or a crate for potty training
And because they are tiny, make sure you don't drop them or let them jump off high surfaces eg some couches, tables, beds etc..
Hope this helps and good luck finding your pooch