Doctor costumes are as cheap as they are unrecognizable, as they are just clothes (unless you have show-specific props). A couple years ago I went to a Halloween party as the 10th Doctor. A lot of people thought I was just dressed up really nice.
If you are needing some ideas for costumes, here's a list of items that can help for each generation:
1st Doctor: A walking cane, white wig and a smug sense of superiority.
2nd Doctor: A bowl haircut, slide whistle and the eyes of a madman.
3rd Doctor: A frilled shirt, a yellow roadster and a love for techno-babble.
4th Doctor: A long multicolored scarf, a big white-guy fro and insatiable love of Jelly Babies.
5th Doctor: Brainy specs, a stalk of celery and an obsession with Cricket.
6th Doctor: Stolen Technicolor dreamcoat, 2nd generation white-guy fro and piss-poor writing (televised stories only).
7th Doctor: A question-mark shaped umbrella, a Panama hat and a look in your eye that states "No Mr. Bond, I expect you to die".
8th Doctor: A bewildered look, Victorian-era clothing and a history that is mainly passed down through oral tradition.
9th Doctor: A leather jacket, a thousand-yard stare and desire to see the fantastic in everything.
10th Doctor: Anything you can get your hands on.
11th Doctor: Anything you deem 'cool'.