Coroplast If I have a C&C cage can I just use coroplast?

It would be OK I believe, and many have done it that way, but your Guinea Pigs may climb out at 1 point. Cubes will also make it easier to contain some more mess when you need to put hay and the like into their cage.

Look into closet racks. They have them in the storage section at Home Depot. Ask them if you can earn some extra money.

If your parents are concern about the cost of the cage, will they be willing to pay for vet fees when your piggies get sick?

If they're worried about the cost of the cage, you'd better have them investigate veterinary costs before you actually get a guinea pig. Regular dog and cat vets won't do, as they get almost no training in guinea pig care. Exotic vets are few and far between, and expensive.

If a guinea pig is healthy, the cost of upkeep isn't too bad -- the cage set-up, hay, food and bedding. But if a pig gets sick, the cost of the care can run to hundreds of dollars and many hours of handfeeding a pig who's determined not to eat.

I'd suggest that you think about a pet that requires less money and care until you're old enough to pay for it on your own. Then, a guinea pig (or preferably two) will make a great pet for you.

I made a 2x4 C&C for 50$. My fleece bedding cost another 30$ but I have so much of it and many changes. I stretched my fleece very far and still gave tons left over.
Honestly, choroplast is more expensive then cubes where I live! If you want to cut costs look into getting JUST cubes and then use a shower curtain and fleece for the base.