I like Corel. Some use Inkscape. There are many others out there though. Just make sure which ever one you go with you can save or export as a DXF. Not all versions of Corel allow that.
That's the cheapest one yet. It works okay "for the price" but it's no Corel. But if you're new to programs like this, it's okay to start with Inkscape. Corel has a lot more bells and whistles but if you don't know how to use them there's no point in paying for them yet.