if you are talking about autocad & not cad than $100 is not enough as autocad is between $1000 to $2000.
he is right auto cad is about $1000 for standard edition and $3000 for a bussiness edition and no you cannot download it for free because you have to register the auto cad software online so using a illegal version is out of the question sorry but you are screwed auto cad is not 100.00 so if you think it is then you are about to purchase a illegal version
Then again saying that... You could download a copy of the autocad software and Evaluate it... try google.com there may even be a copy of it... or something compatible on http://download.com or http://cnet.com or possibly even http://soft32.com hope this helps. Also some people may obtian "cracked" versions of the program or use illegal serial numbers or codes. This activity is NOT condoned... But i do believe you may exercise your right to evaluate the Full version if you want.. dont hold me to this tho.