Bohema Artisan and Vintage Boutique (6152 Ridge Avenue, 215-482-5393) in Roxborough will take your old costume, and allow you to pick out a different one from its costume section.
The owner assigns each costume a number of points and you can trade for something of equal value, or add cash if what you want is worth more than what you're trading in.
They have costumes for adults, children and even dogs! Use your creativity to find props and accessories and you'll end up with a brand-new Halloween look at a great savings.
Here's a link to the store's website: Bohema Artisan and Vintage Boutique.