How do I persuade my parents to get me a guinea pig?

I'm trying to convince my parents to get me a guinea pig, I've read up about them and know what food they eat, how to tell when they are sick, what bedding to get, what cages to buy, and I also know that it's better to get 2, so if possible I want to try and get my parents to buy me 2. The reason they won't buy me one is because I had a hamster when I was 8, and I didn't take care of it, but I am older now, and this time I will clean it and feed it. I'll pay my whole allowance for it if I have to. I've told them this, but they won't listen! And they think that a guinea pig will cost too much. Any tips on how to convince them to buy me one? (or 2 if possible)