Right now, I think TurboCAD is probably the best low-cost alternative. It's $60 US for a 2D-only version, $200 to add 3D functionality.
Have you used TurboCAD, and in particular the 3D? How good is the 3D? For example, can it do a Les Paul styled carve top?
I've found that things like Sketch up are terribly limited.
