A cheap airsoft BB gun is not hard to find these days. Any Wall Mart or K Mart will have one, and there are many online warehouses that sell them. There are three types of airsoft guns: spring powered, gas powered, and electric powered, and all three have cheap versions. The cheapest guns can be bought for around $20, and are great starter weapons for those just entering into the sport.
Airsoft BB guns first came into being in the early 1980's, and were manufactured as replicas of actual weapons for those who desired to hold and shoot their favorite gun without having to purchase (illegally in some cases) the real gun. They were designed to match weight and firing rates of their real counterparts and due to the detail of their construction sold at a premium.
With the advent of sport shooting with paintball BB's cheaper airsoft guns are now made. They have much less firepower and round capacity than the more expensive models, and stray far from the design of their real predecessors, but they still give the user a feel of what shooting an airsoft gun is like. Most cheap guns come with a small package of BB's, a battery (electric models), and limited gear like goggles or a face shield. All of the guns have a bright orange tip to denote replica guns, and come in various colors and sizes. If you would like to see what an airsoft BB gun is like before making a major investment in a higher end weapon, try out a cheap gun and see what it's all about. If you like it, you'll want to try one of the more expensive models to see what serious fire power is like.