Crazy 8 and The Children's Place at our mall offer discounts if you say you're buying for an Angel and show the angel ornament as proof. At crazy 8 we received a 25% discount and it was 20% at The Children's Place.
$15 for a child's dress is very reasonable, especially for some of the adorable brands that Target has out there right now. The Trend D line is ADORABLE and so is the Harajuku Mini line. The fall line of both are on clearance at Target right now, and the winter/Christmas lines are on sale. You can save an additional 5% at Target if you link your debit/checking account to what is essentially a prepaid Target card. It links to your banking account and it gives you the full benefits of having a Target credit card without having to actually go on credit or make payments.
While dresses are adorable to buy for a 6 year old, not all girls are dressy and she may not ever wear skirts/dresses or even like them. If you're looking to go for more of a versatile gift, I would go with a nice pair of dark denim that could be worn to school and for dressier occasions, the dark denim will last through more washes and could potentially look just as dressy as a cute dress. I would get a couple of long sleeved shirts, or a hoodie/sweater, and definitely a jacket. Target generally has a selection on clearance right about now, as late winter and spring lines start to make their way out. Depending on how cold winter is in your area, a hat and gloves and scarf set might be wonderful to add on too. Socks and panties are always good ideas too, sometimes these essentials are few and far between for families.