If you need to convert AutoCad DWG files to other formats like PDF, you won’t typically need to pay anything. There are a few free tools that work great, like the following online service:
That site offers quick and painless, free conversions of multiple file formats, including DWG to PDG.
If you need something offline, I’d have to recommend the cross-platform draftSight software. More about that here: http://www.3ds.com/products-services/draftsight-cad-software/
Not only does draftSight do conversions, it’s basically free, and it’s a very powerful design software to boot.
A quick note for Linux users, it’s available for download as a .deb file so once downloaded, it can be installed from a terminal like so:
sudo dpkg -i draftSight.deb