I looked around briefly for a Robin costume, and while there were a few to choose from on eBay, and they weren't terribly expensive, I decided to make one my own. Going by a couple pictures of Robin that I found online, I started searching around my house for what I needed.
I already had a plain long sleeve red shirt that I had picked up at Walmart for $3.50 - it was part of my "I'll eventually do something with this" stash. I didn't have the right shade of green for pants, so back to Walmart I went, and found a pair in the girl's dept for $3.50. I raided the Goodwill bag, and found a yellow tshirt that I decided would work perfect for the details on the costume. And finally some black felt to finish out the details.
Everything is attached using Heat n' Bond. I had every intention of sewing everything down for extra security, but my sewing machine and I weren't getting along, so I didn't. It held up well for the most part, although the R disappeared before the end of the evening.
For the cape I just free hand cut it out, using the tshirt as a guide, and attached with velcro on the shoulders.
I was pretty pleased with it - not too bad for my first attempt at making a costume!