Ok, as a student, I've thrown fancy dress costumes together fairly cheaply...
GI jane/army chic- easy peesy, done this one twice. first time got an oversized camoflage t-shirt from the market for about £3, and stuck a belt round the waist to give it shape. wear with jeans/leggings/cargo trousers. 2nd time I bought a camoflage dress from new look (they still have them) and did a sexier version. put dirt on face. camoflage hat from market (or army and navy stores) about £3ish? dog-tags about £2
Pirate- stripy top + whatever the heck you like on the bottom. kiddies pirate set including gun, eye patch etc about 2-3 quid
schoolgirl- duh! black or denim skirt + white shirt + tie + freckles drawn on face! pigtails and long socks optional!
roman goddess- use a white bedsheet and fashion into a toga. gold jewellery.
mummy- buy a couple of rolls of bandage and wrap yourself up from head to toe!!
cowgirl- check pattern shirt, denim skirt, and cowboy hat- about £3 on market or £4-5 in shops like internacionale. gun optional
hawaiian- loud blouse/top, grass skirt (with something underneath to hide your modesty!) flip-flops
moulan-rouge- (not done this myself, not for the faint-hearted!!) corset, sexy skirt/no skirt
suspenders, feather boa etc
hope this helps!! I'll put my thinking cap on and try and come up with some more!! this is fun- it's reminding me of the days when uni was fun and i didn't have so much work that I wanted to kill myself!! lol