I can't really tell you how much $ you'll spend; that all depends on your mad shopping skills.

I can, however, tell you what I bought/am buying for my DS6 for school. He is getting:
-24 prs of socks (ALL the same style this time)
-No new underwear yet (he has scads of them and I'll buy more when he outgrows them-usually 10-15 pair)
-3 pairs of jeans
-1 pair khakis and 1 pair darker "dress" pants (usually Navy)
-2 pairs of black or grey sweatpants (He wears these on gym days)
-5 white long sleeve tees and 5 black (he loves to wear his regular tees, so this makes it possible in winter)
-3 or 4 printed long sleeve shirts or polos
-1 white turtleneck, 1 black or navy
-1 nice sweater (he HATES them, so I only really make him wear them for pics)
-2 zip hoodies
-1 winter coat
-hat, gloves, possibly a scarf
-1 pair new sneakers (that's all he ever wants to wear anyway)
I don't tend to buy tons of stuff at the beginning of the year because he outgrows it too quickly. I mostly just buy enough to get through the week without having to do laundry (in case it comes to that). Hope this helps. I don't know a thing about shopping for girls, though. Sorry! Good luck!