If you are very tall, an off the rack Halloween costume like what they sell at Party City is not going to work. However, with a little creativity, you can still put something great together that is affordable.
1. Tall people always stand out in a crowd so play that up by picking a larger than life character to impersonate: think super heroes, classic movie villains, and extremely famous people
2. Theatrical makeup, wigs, and other props (like a whip) will fit you from regular stores. As a starting point, decide how you will use these elements and then plan the rest of your outfit around that.
3. Even if you are seven feet tall, you can buy a custom body suit or leotard from one of many Chinese manufacturers for under $50. These are useful for creating superhero costumes, as a skin color base (like blue for a Smurf), or being modest when you wear a flesh colored version under more revealing costumes. Just be aware that shipping takes a long time so order at least a month in advance.
4. Close enough is good enough. Ultra tall clothes are expensive, so work around hard to fit clothes that you already own like pants, floor length dresses, and long sleeve tops even if they are not exactly right. For instance, when I was Marge Simpson instead of buying a floor length green dress exactly like hers, I used a green midi dress that I already owned and after adding makeup and accessories no one noticed that it wasn’t exactly right.
5. Stay in character. Good acting and confidence will sell an otherwise mediocre costume. If you are a robot, walk stiffly. If you are a villain, glare menacingly once in a while. People will remember that much more than the fact that your superhero leotard was not the right color.
I hope that helps. If you have any other tall costume hacks, please add them in the comments.