How much money does it cost to take care of guinea pigs?

I am 12 and I want to get 2 guinea pigs during fall ,but I don't know the the cost.I will be using fleece as the bedding and will make my own 2x4 cage.I will be also using a litter box so I will need some bedding.I already have about $150 ,but I will sell some stuff to take care of them.My parents would pay for some of it too. THANKS ^^