To give you the "quick and dirty", it all depends on what your intentions are. If you're looking to have parts CNC'd, then 3D modeling yourself CAN be a huge time/cost saver.... the caveat here is that you must make the model "correctly" (just because it's the right shape doesn't mean the CNC will be able to make it). So for someone who isn't doing this for a living and/or doesn't have the experience, it is usually better to have the CNC shop do their own programming off your 2D drawings.

That gets to the next point... what level of "designing" are you looking to do with the software? If you're looking to make professional level drawings, then AutoCAD is probably your best bet... depending on the complexity of the part.

I'm not sure how much it costs, but you may want to look at Autodesk Inventor. I'm a mentor for FRC (First Robotics Competition) and they're pushing Inventor as a good entry level 3D program for the students to pick up... so i'm guessing it's very user friendly. I haven't played around with it much because FRC also provides a Pro-E Wildfire 5.0 license that I use for the robot design; and just due to lack of time I haven't had a chance to play with/learn Inventor. I believe its cheaper than SolidWorks... but don't take my word on that.