If you are running your computer system on the Windows operating system anti-virus is all but a necessary requirement of using the computer. The cost of anti virus software is best looked at when considering the consequences of not using it
1. Lost Reputation
When you have a virus on your computer it usually does not just stay there many viruses or worms will attempt to replicate themselves and this is usually accomplished by means of email. The worm will use your address book to email itself to everyone else. This not only makes your own system slower, but can also affect your customers, clients, vendors, and employees. If you infect other computers because of your negligent practices others may equate that with how you work on other aspects of your business.
How much is your reputation worth? How much is it worth to keep?
2. Lost Time
Viruses can really destroy your productivity, and if on a network can spread rapidly throughout the entire work place or home. Depending on the virus it may slow the system down to the point of being unusable or it may keep repeatedly crashing for no other reason. Many viruses will stay on your computer for months slowing it down and more and more will "gather" on the computer.
How much do you value your time? Remember everyone is given 24 hours a day, it's up to you to deem how those are spent.
3. Lost Data
Many viruses do not just slow the computer down they also destroy data. Some delete the whole hard drive, some corrupt certain files. You may find that file names randomly change or "weird" things happen to files and directories.
How do you value your data? Is everything backed up properly? Are you sure the backups are up to date and in a readable condition?
The Solution
Many people hope that nothing happens to them because of the high price of the available products. However, there are quality low cost alternatives such as AVG. AVG also comes with two years of included free updates, instead of the more usual one year.