You don't have to spend a lot to get a good costume.look out some old clothes for ideas.
Look out a black dress now all you need is a witches hat. Wich you can pick up cheep near haloween and even get decent ones out the pound shop you can even pick up witches brooms hear to
For this you can use old clothes you don't need any more or pick somthing up at the charity shop rip the ends to make them look old you can go as a tramp add some make up to this and some fake blood and your a zombie
If you have a black suit just add some shades and look around the charity shops for a trilby or fadorah hat and your one of the blues brothers
Another great cheep idea is the vampire because you can wear anything you can buy cheep make up kits and capes if desired at halloween you can go to the pound store for these to use the kits to make you're skin paler and add some blood drips round the mouth