I am also planning on making a Hit Girl costume, but I'll be wearing mine! I'm probably not too much taller or older than Chloe Moretz. She's like 13-14 (I'm 16)?
Anyways, this is what I was planning (and thanks for all of the links and names of all the crap she was wearing! I would have never been able to find the items otherwise.):
For the body-piece, a metallic purple large, loose zentai (full body spandex suit)
many found on eBay for less than 35 dollars. I was gonna cut the hands, feet, and head off and stitch in the various lines on HG's jacket and pants, as well as add a center zipper. It's a little lighter than HG's but it'll suffice.
For the wig, since I couldn't find the right cut in the right color, I was gonna style a longer, correctly-colored wig.
The cape and kilt will be made from fabric from the fabric store (like JoAnn's). The cape will attach directly to the neck of the zentai. I will make my own kilt from a purple plaid dyed a little darker with coffee.
The boots are fairly easy to acquire at places like Kmart or Target.
I will construct the kneepads from cardboard and stitch them into the zentai.
The military suspenders: 2 black backpack straps attached to a black belt (martial arts) with taped on knife sheaths. Grenade made of plastic yellow egg with keyring and nail.
The pink utility belt: a plastic batman utility belt painted pink with that Krylon Satin "Ballet Slipper" #53526 mentioned earlier. and 2 chapstick tubes with electric tape wound at the ends for those belt tubes. (The front 2 extra pockets cut off and painted black to add to the suspenders). The HG logo glued on the center.
The doubled-sided spear thingy: thin black tube with cardboard cutouts wrapped with foil.
And I think that's it. I added it up and it's roughly $110 with the cheapest versions I could find. Many things my dad has (like the weapon parts and stuff). I'll add if I remember anything else.