Is this on a CD? If so, there should have been only one disk. The original installation instructions would have prompted you for a serial number and an authorization code.
Low cost? There are a few freebie CAD programs available for downloading off the Internet but AutoCAD isn't one of them. There are also 30-day trial versions but that's not going to help over the long term.
Did you get a manual with this software?
Did you try contacting the person you bought the software from?