If your Halloween costume was something special, one of a kind, or expensive, and is still in good condition, you might consider selling it. This will allow you to retrieve some of the money that you spent on it so that you can put this amount towards this year's Halloween outfit.

Consignment stores
A consignment store is a bargain hunter's dream for Halloween costumes and other deals. You give the store the costume, and they do the work of finding someone to buy it. Once it sells, the store keeps a portion as their fee and you get the rest.
Online Auctions
For people who prefer to sell their costumes themselves, online auctions can be a great option. You will receive a much broader audience than your local store or garage sale. Always read the terms and conditions of the particular site before you post an ad.
Garage sales/newspaper ads
Of course, if you want to sell your Halloween costume yourself, the traditional avenues of selling it at a garage sale or with an ad in the newspaper may also work. Keep in mind that you will usually receive less money for the costume this method, though this method may be easier if you were already planning a sale.

Donate Your Halloween Costume

If your Halloween costume is still in good shape, you could consider donating it. There are several places that would appreciate your donation. Here are some places that might be able to give your Halloween costume a second home.

Women's shelters
Many of the women staying in shelters have kids with them. And when times are tough, extras like a Halloween costumes are not going to be in the budget. This is a special place to donate kids Halloween costumes to so that another child can have a chance to trick-or-treat. They may also receive adult costumes as well so that mom or older children can join in the holiday fun.
Children Hospitals
Staying in a hospital long term is not much enjoyable, so the staff at children's hospitals will usually look for ways to cheer their patients up. Dressing up for Halloween, or even a day of silliness, can make their patient's times more enjoyable. Some hospitals will also allow children to trick-or-treat about the rooms of the hospital on Halloween night.
Churches (some denominations)
Some churches will celebrate Halloween, or have a church friendly alternative where kids can wear their costumes. Families in need may approach their church first for help before looking elsewhere. You will want to phone the church beforehand to make sure they would receive the donation, and use this for your family friendly costumes.
Charities
Many charities like Goodwill and the Salvation Army will welcome donated articles. In most cases, these organizations will sell your costume at their discount stores to help raise funds for the different programs that they run.
Hospice retailers Hospice services often run small stores to raise money for their staff members. These people often do a lot with very little, and they will take usually any type of offering that is in acceptable shape.

There is always people out there that will be interested in your Halloween costume. Taking time to locate them can help you recover your money or make someone else's Halloween brighter.