How to convince my mom to get a hamster?

I am 13 and when i was 7 my mom got 2 cats both male named Tiger and Oreo, then i received another cat Carly in January 2009. later that year my dad (who lives with my step family) got a Yorkie named Roxy who i love but have no influence over. i love all my animals and would own day love to be a vet. Even before i got Carly i wanted a small animal and i was going to get a bird but my mom felt more comfortable with another cat so we got Carly. i don't regret Carly but i really want a hamster now. I have done must research on alot of caged animals including turtles and even Sugar gliders but hamsters seem simple easy and cheaper. i always wanted to buy a exercise ball for some reason and gerbils and mice are out of the question. i love all my animals dearly but my mom says that's it! i respect her decision but i have a right to fight for what i want, don't I? I would never mistreat an animal and would never buy an animal behind my mom's back. i really want to prove to her how much i want this animal. When i wanted my dog i did tons of research on toys, health concerns, breeds, etc; i even made a booklet for my step family. My mom would never get a dog but a hamster is, while still a responsibility, much easier to handle. i am currently doing much research on Hamsters on and off Yahoo answers is there anything else i can do? Please respond! Thanks guys!