I have a handmade cage and I used coroplast; but the plastic had been never seen wet or dirty, because there are other layers which make the floor thick enough. I use old towels. Therefore you could use also a cardboard... You should cover only the edges with an adhesive sheet or something else, because the piggies might nibble at it.
For saving money I recommend you to use old newspapers and leaflets from supermarkets for the absorbant layer underneath a simple piece of fleece fabric. The fleece will be washed once/twice a week (buy 2-3 pieces of fleece) and the newspapers work amazingly and are totally free. You need only to remove wet papers once a day.
Then you can cut off pellets... they are not necessary and are also the cause of a lot of health troubles (and the vet is always expensive). You can give your piggies just 5-10 grams of good pellets a day, as a treat. In your district there must be a lawn, also a private lawn: go there and and pick up the more grass you can; grass is the best food for piggies and is free. It can stay into the fridge for 1-2 days well closed and a bit wet into a plastic bag.
Hay: if you go to a farmer and can buy the hay for horses in little bales that is cheap and very good. Otherwise you can buy normal bags of meadow hay online which is cheaper than at the shops.
Saving money with food this way does not mean that your piggies will receive poor cares. It is just like the food for us humans and children, a fruit, a veg or an egg are cheap, but healthier than any take-away or processed food.
I upload some pics, maybe they can give you some idea...(there are a lot of recycled objects which can become useful for your piggies)
the fleece I use is a simple piece of fabric:
grass is their main food (and their health is good, so far)
recycled materials:
