Type Airsoft Gun
These are the cheapest airsoft guns you can get. An internal spring shoots the pellet from the gun. They're single shot guns which means you'll need to cock the gun each time you want to fire one of the plastic bbs. This can get annoying at times but these guns can be purchased for just a few dollars.
Gas powered models add one immediate feature - automatic fire. With a gas powered airsoft rifle or pistol you don't need to cock the gun each time because it has a gas charge built in. These are far more powerful than most spring airsoft guns and slightly more expensive. This extra power comes at an added expense - the gas needs to be refilled. This can be achieved with co2 cartridges or a portable hand pump. For the added expense gas airsoft guns definitely feel more like the "real thing".
And finally the electric airsoft gun. Also known as AEG (Airsoft Electric Gun) these are the top of the range when it comes to airsoft guns. Each AEG has a tiny but very powerful motor inside which powers the firing mechanism. There is still a spring inside an AEG but this is controlled by the electronic motor to achieve high rates of fully automatic fire. One of the really neat things about this type of airsoft gun is that it can be loaded with miniature paintballs. A word of warning however - if a paintball breaks inside an AEG it can clog up the entire firing mechanism meaning you'll have to strip the gun down and clean it.
So with airsoft guns it's best not to focus on the word cheap. What you're looking for is value for money and the features that suit you most. You can buy an airsoft gun for less than $20. That doesn't make it cheap - just very good value for money. Regardless of the cost of your airsoft gun once your friends see your new "toy" they'll want one too!