Are you looking for the WHOLE costume, or just the uniform?
The uniform itself on eBay is around 70-100 dollars PLUS 50 shipping & handling. A good wig is from 18-30 dollars. Shoes also in the same price range, and at least $8 for a good pair of socks. Some costumes may not come with the accessories either. You total is at LEAST 164 dollars. Without basic make up if you don't have any.
Let's not even go into contacts, since you might not need them.
Two costumes I liked that come with the accessories:
http://www.qqcosplay.com/VNT/vntyukblk.html
http://www.399cosplayshop.com/japanese-anime-cosplay-costumes/vampire-knight/cross-yuki-cross-academy-female-day-class-uniform-costume.php
I chose the Day Class ones, but they also have Night Class, and they also should have them for boys.
The other answers may look like they have cheaper costumes, but remember, $50 for shipping and handling. These two aren't eBay stores. QQCosplay adds the shipping fee to the price already, but 399cosplay adds a $35 dollar shipping fee, but it's better than 50.
Wigs (Don't skip them. About no one has hair like an anime character):
http://www.amphigory.com/wigs_fy_always_sexy.html
I think that's a good wig for Yuki.
I think the best thing for Zero would be to get this wig and go to a professional and trim it.
http://www.amphigory.com/wigs_a_passion.html
But you can also look around on http://www.cosworx.com . They have good wigs as well as shoes if you can't find them at payless.
Do keep in mind that it ususally takes 4-6 weeks for shipping. You should have enough time this time, but you may be screwed if they end up with a delay. So if you want to buy a costume, try and order it maybe six or seven month's before hand.
EDIT: Apparently the cosplayfu one DOES include free shipping, but it seems to be missing accessories, especially that Yuki Cross one.