sub, that's not how it works.
it's like this.
Each major brand has several different models.
The cheapest
a little better
even better
a lot better
THE BEST

don't buy the cheapest (unless it's an on sale last year's model)!! don't be tempted. go for a little better, even better or a lot better.

the bikes that range from $500 - $1000 are the best bang for your bucks. (a little better, even better and a lot better usually can be found in that price range)
now these are road bike prices, but i am sure it's very much the same with MTB's

further, if you buy her a bike that is THAT much bigger than she needs she will NOT enjoy riding, she will suffer pain and you will have wasted your money.