3d modelling software for case design

Generally, you need software that can export STL files.

Solidworks: $4000 + $1300/year

ProEngineer: $5000? + something per year?

Autocad Inventor: $5200? + something per year?

Google Sketchup: Free or $500, but can't export STL directly. I've seen mention of a plugin that can convert Sketchup files to STL, but I haven't used it myself.

(From Peter Gibson) Blender: Free, but alleged to have a steep learning curve.

SolidWorks

Take a peek at Alibre Design. It's more affordable than the other options, and has a free trial. I bought it recently and I'm very satisfied. Not only it meets all of my needs, I compared it to Rhino, SolidWorks and others and found it way better, both in capabilities and user interface.

http://www.alibre.com/products/mainpage.asp