See if you can show your parents how much pellets are per day, and then compare the cost of the daily cost of the veggies that bpatters mentioned for pelletless stone pigs.
Going off the prices of bell pepper here (as an example) one large pepper is about a dollar, so if you're feeding a quarter of a pepper daily (between two pigs), it's 25 cents. Using @
bpatters numbers, KMS pellets fed appropriately at 1/8 cup per pig per day are 12.5 cents a day. I wouldn't use bell pepper to persuade your parents, because you don't want them to stop buying them for you, but look up prices of some of the other veggies that bpatters mentioned, and do the math for them. Veggies are expensive and cold hard numbers might open their eyes to your argument better.
And if they don't want to pay for the pellets and you don't have the money right now, consider seeing if they would work out a payment plan so even if you don't have $45 right now, you could pay them $5/month or something until you've paid them back? You could even offer to pay a small amount of interest on the loan, like you would in the real world. There must be some way for you to get $5/month.