How do I convince my mom to get me a puppy for x-mas?

So, a family friend of ours has 4 boxer puppies for sale for $450 each. However, she said I could get whatever puppy I wanted, for only $350. I want the runt of the little, she is a light brown and a fighter. I'm in college right now, in the dorms, but my boyfriend and I will be getting a place come January. By the way, the puppies will b ready for new homes on x-mas. I've had a puppy before, and I know I can take care of it, but for some reason she just isn't going for it. What do I do?!