Cheap method to drill neat holes in plastic casing?
A hand-held tapered reamer (http://www.google.com/images?q=tapered+reamer) is a good tool to have for this.  A "step drill" bit is even better.

For starting holes, a small hand drill (non electric, http://www.google.com/images?q=egg+beater+drill) gives much better control than an expensive electric drill anyway; try a flea market or junk store.  You can also melt a starting hole for your reamer hole with a hot nail or a worn out soldering iron tip in a pinch.
A small hex shank drill bit can also be used by hand mounted in a bit-driver.

For rectangular holes, a hand nibbler (http://en.wikipedia.org/wiki/Nibbler) is great, starting from a hole made by your reamer or step drill.