AntiVirus is a very high volume niche, and due to the fact that the consumer usually requires an immediate fix, the conversons are high. If you are looking to promote software, and want a niche that can be scaled to high $$$ a day then it's a great place to start.
If you're just getting your feet wet with paid traffic then it can definitely get pricey, but depending on the offer, and how much support your network is willing to give, you can see success out of the gate.
What kind of traffic are planning to work with? Paid or Organic to start?