You can buy a Dr. Horrible costume online for $89 or make one yourself for less than a quarter of that. Even less if you already have some of the pieces in your closet or garage. I'll tell you what you need, which pieces you'll have to make and the extras I would do for an adult costume. (Maybe I'll be Dr. Horrible next year.)
- a long sleeve plain white shirt, preferably a long sleeve t-shirt type
- white scrubs (just the top) or a lab coat. I got the scrubs top at Walmart for about $12.
- grey rubber gloves. These are an adult size from Home Depot. They'd obviously fit an adult much better.
- a big water blaster. This can be spray painted silver and even filled with colored water, if you want.
- tan or grey pants. Dockers are great, but even riding pants would be good since they get tucked into the boots
- rubber rain boots spray painted grey or white. Or you can add painted cardboard cylinders over your shoes to make faux boots.
- cheap, plastic safety goggles
You'll wear the long sleeve shirt under the scrubs or lab coat. If you want, you can use fabric pens to make the pocket insignia. I used a button instead. Wear the gloves over the long sleeves. Slacks tuck into the boots.