IF (only if) you don't need in-depth 3D modeling capabilities, I agree with borgunit: AutoCAD LT is a good alternative also. At my last job, I ran LT 95% of the time, for over 4 years. At my previous 2 jobs, it was all I had, for 2.5 years. As of 3 years ago, it was about USD800; I believe it is still under USD1k.
LT is not, however, a truly 3D software. But you can give z-dimensions to things and "fake it" for light-duty 3D functions.
Engineering is the practice of the art of science - Steve