I would like to adopt a kitten/cat but how to persuade my parents?

A lady I know has 2 cats that just had kittens, so she now has 11 little sweeties :) I have wanted and have been thinking about cats for the past couple years and I know Noe I am ready to take on that responsibility.I had rabbits before which I took good care of and they passed away a couple years ago and vpi had a big Akita when I was 10 and he was to big for me to handle so we couldn't keep him but Now I have 1 cocker spaniel which I payed for and I care for and love, my neighbor had a cat before, and he did fine around it, he is really gentle and a wonderful dog, and by the way wew have mice in our garage again so that would be a plus for my parents about having a kitty, I would help out in the cost with all the money I get, I have about 120 dollars right now and I have a dsi and iPod and stuff which I could sell to help a little, and I could probable get a job soon to I'm 14 please how can I show them I am ready for this and that I just want another friend and companion and that I can put all the money I make towards it?,