If you're going to get one, I reccomend you adopting one from a shelter. For one, they are cheaper(usually $5-25) & often a lot more friendlier. The ones from pet stores range anywhere from $30-70, & they are not socialized. Guinea pigs are like rabbits, they poop & pee a LOT. Which means they smell & you have to change their tanks like 5 times a month. I advise you too keep it in a cage like this, http://funadvice.com/r/1660hfe66k5 The cage is probably around $75-100 but you may be able to haggle & find a cheaper one on craigslist/amazon/ebay. The fleece bedding is probably about $15-25, but you have to change it often. I do not reccomend any kind of wood chip beddings. It causes respitory problems & shortens their lifespans, slow & painful death pretty much. Their food bowls/water bottles range anywhere from $2-15 depending where you get it from(I find walmart a ton cheaper than any pet store, for any supplies). Their food is pretty cheap, depends on how big of a bag you get. They're pretty expensive when you're first buying everything, but after your settled you're only paying for bedding & food.
You should also have plently of money to buy them treats, depending on the breed. My pigs get fresh carrots every day and cubcumbers, romaine and hay weekly. Toys are also a must, guinea pigs love to be interacted with, they are kind of high maintence so when you aren't interacting with them they shouldn't be bored in their cage they should have some things to play with.
Key word "pig". They will eat anything, non stop if you keep feeding them and health problems can arise from overfeeding or feeding them foods they shouldn't be eating. My former MIL kept feeding hers cheese puffs "because he loves them"...and it died. If you do get one, be careful what and how much you feed it.
I keep mine in a hutch outside with old newspaper lining the floor, and hay in the sleeping compartment. They are fairly cheap to look after in general. They can be fed on lettuce, carrots, tomatoes and bags of Guinea pig food. They make great pets and are easy to tame.
I'mma say $10 - $20 for the actual guinea pig. Then $30-$50 per month for treats, toys, food, bedding, etc. Don't forget your cage... $40 - $70. Though you can find a used one on Craigslist for cheaper. & of course any vet bills that you could have.
I also advise looking on sites such as Gumtree for hutches or cages.
Around $40-$70 for the guinea pig and cage i really wanted one...