How about AutoSketch?
This is by AutoDesk the same as AutoCad but their low end version.
Their propaganda says that you can edit a DWG file with AutoSketch.

Amazon lists it for $118.99
http://www.amazon.com/exec/obidos/tg/detail/-/B0002MAG0A/ref=ord_cart_shr/105-3379851-6261259?%5Fencoding=UTF8&m=ATVPDKIKX0DER&v=glance
http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=2753027

You can get AutoCad LT for a 30 day free trial from Autodesk.
http://usa.autodesk.com/adsk/servlet/index?siteID=123112&id=2498418

Amazon lists it for $809.99
http://www.amazon.com/Autodesk-0572...s=software&qid=1194817907&sr=8-1