Comment 1 for bug 503784

There is indeed no command line option to export to .dxf (i would think that's because it's implemented as an extension) but it should be easy enough to do with a little scripting.

Option 1:
inkscape filename.svg --export-ps=filename.ps
use share/extensions/ps2dxf.sh (which in turn uses pstoedit)

Option 2:
It might be possible to do it using the FileSaveAs verb but I don't know exactly how
see http://tavmjong.free.fr/INKSCAPE/MANUAL/html/CommandLine.html