How do I convince my parents to buy me a horse?

I am 13 and am an intermediate rider. I want a horse more than anything else in the world and my parents seem to know that I am not going to quit. They say it is about the money but the are leasing a horse for me for only, like, $100 less than the monthly cost to keep one. I already know where I would board it and there it is VERY cheap but good quality and they provide the feed. I already know which horse I want. It is a horse from my lesson barn (the same barn I would board at.) Nobody else but me rides her and we have bonded very well. I want a horse more than anything. I understand there are additional costs like vet, farrier,saddle, bridle, halter etc. Please help me!!! I am 250% sure I am ot going to EVER quit.