How can I persuade my parents to get me a pony?

I have been riding for 6 and a half years and have helped out at my local yard for 4 years. I know a livery yard that I can keep the pony at for a reasonably cheap price. My parents know that I love horses and are sensible and mature. They are able to financially cope with buying a pony, but I think they are worried about spending so much at once. They know that I am a good enough rider and deserve one. I have made a presentation on why I should get a pony, a leaflet, spreadsheet on the costs and I cam going to make a cake for them to eat whilst watching it. I have been planning thsi for ages and I am worried that if they say no I won't have another chance to persuade them. Any ideas? I am doing the presentation this Friday (21st March '08). Thanks and all ideas are welcome!