The idea is free, and the costume is free, provided you already have a white t-shirt, purple paint, and paint brushes. Or purple fabric markers. But the best part is that it is easy. You too could be a one-eyed, one-horned, flying purple people eater.
1. Find an old white t-shirt you don't care about. If yours is like mine, it's your husband's old cell biology conference t-shirt with a big nerdy picture and a pretty blue stain on the front. In which case you turn it inside out and backwards. Voila, blank canvas.
2. Stick something up the shirt so that your paint doesn't bleed through and stick the front and back of your shirt together. You could just put the shirt on and paint it while you're wearing it, but as entertaining as that would be to watch, I wouldn't vouch for your results. I recommend cardboard. I used an empty cereal box.
3. Paint. I free-handed mine, but you could definitely use stencils. But then you don't get to make up your own font, which would be a real shame.
4. Admire your handy-work. A hearty "Ta-da!" would not be out of place here.
You're so welcome ; )
p.s. I'm sure that my purple person was definitely inspired by something specific - some odd cartoon show, perchance? - but I really have no idea where I came across said inspiration. Bonus points if you can figure it out.