Not sure about for software, but a friend of mine just got a copyright on a book he wrote and it was 50 or 55 USD, something like that. So you could put a legitimate copyright on it and then give it away for free. But everything I've seen, has been mentioned that you can provide a LICENSE file with it that has whatever license you want for it. You could always use this;
http://www.wtfpl.net/ 
