You can expect to pay around 10.00 for a guinea pig.
Then you need a bottle each, these are usually 99p each.
Food for the month is 8.00 a bag, the best one for them is Supa Guinea Excel and you can usually get this from pet shops or vets.
Straw for them to sleep on and eat.
Shavings for them to play on.
Have some fresh oranges,celery, apple, parsley, broccoli, cauliflower, carrots (every other day)
You can also give them bits of banana occasionally, don't give them potatoes as they are far too starchy.
Also, when you get them, you will have to check them for mites, they are very very small and when you part their fur they bury under the fur, the vet will give you some drops and once they have been applied they are all gone within an hour. These mites are species born, which means that they cannot be passed onto humans or any other animals.
If you are going to keep them outside (though having them inside can increase their life expectancy by 2-3 years and also reduce the risk of then getting respiratory probvlems), you must put them in a hutch on legs and make sure that you have a cover to keep the wind rain off etc, if you can't keep them indoors, but have a shed or garage (unused garage) that would be ideal.
Any more questions, please shout and i'll be happy to help