If you go to a department store and look at the finely knit sweaters (especially Polo) they can cost upwards of $400. You're lucky to find them under $200 on sale. $100 doesn't seem so bad to me. It takes a lot of yarn to make a sweater. Plus, when you're done you'll have a hand-made custom garment.
Just make sure it will fit.
Haven't you ever noticed, how much better a meal tastes when it was made from scratch with love. Nothing beats that.
However, something to consider. A teenager, (don't want to make any assumptions about your dd, but I have teen sisters) may not appreciate what it means to have a hand made garment. Teens are so caught up in designer names these days. She may not understand how much time and effort goes into something like that.
Maybe, just make her a super nice hat or something. It's a lot easier to fit those. Takes a lot less time and yarn too.