Economy works because people have a understanding of the definitions for "value" and "fair." When those definitions break down, economy collapses EOL.
If "free" is in your vocabulary for acquiring property or rights then what you are acquiring is either worthless or its cost has already been paid by a past/future asset or experience. Those who ignore this law will one day be asking themselves about a mysterious negative number/experience that will exactly equal what they should have used in place of the zero they used when they equated it with "free".
An honest estimate of costs IMHO follows:
ClamAV costs: $0 + 1 well educated computer expert user per computer willing to do his/her own work to secure his/her own system without automation or supervision and who has no qualms using other software to filter spam and monitor applications while working + 1 Network administrator who does not want to put security software onto his/her servers or has an alternative enterprise solution already.
Microsoft Security Essentials costs: $0 (for up to 10 machines) + 1 computer user per computer of any expertise level that does not want or need a spam filter solution + 1 network administrator that has an alternative solution for his/her servers and who does not use Microsoft Exchange.
Comodo, AVG, Kaspersky, ESET NOD32 each cost: $10-$30 each user annually + 1 average level user for each computer that does not need or is willing to buy their own spam filter system + 1 network administrator that already has an alternative for Microsoft Exchange or does not use it.
Trend, Symantec, McAfee, Panda and Co each cost $50-$100 for each user annually + 1 user of any level of expertise per computer + 1 network administrator willing and able to configure advanced features to be remotely deployed to all systems.
I reviewed three such tools earlier last year and tested them against an actual virus root kit on Examiner.com. They are all decent options, each with weaknesses and strengths.
http://www.examiner.com/information-technology-in-indianapolis/jasen-ward