If her feet are still growing, I would simply stick with paddock boots and get a nice pair of half chaps for now. That's the best way to save the bank.
If you must go with tall boots though, eBay is a great site that has hundreds of ads up always. I just bought a brand new pair of crownes for $100 plus shipping. But, as horselovin mentioned above, it's really about the care they get. I have an eight yo pair that looks pristine because I'm meticulous about them- and I still wear them every day. And when you do the math, the good quality that lasts a while really pays off.
But, for now while she is still growing, I really think a nice pair of paddock boots and half chaps (they do make leather ones) would be fine. Children's feet grow like weeds!
We are not human beings having a spiritual experience. We are spiritual beings having a human experience. -Pierre Teilhard de Chardin