Every case in Counter-Strike: Global Offensive contains certain skins. Most of them are cheap skins (blue), and the rest are fancier items, though a chance of their drop is low.
This is what defines the cost of each particular skin. A large number of skin drops drops the price (pun alarm) at a trading platform. Accordingly, rarer skins cost more as they are fewer in numbers and Fallout less frequently.
There is a line “or an exceedingly rare special item!” in any item list of a case. It is this entry that means a drop of a knife or gloves.
The chance of the drop is extremely low. The game developers have never published drop probabilities. Based on the users’ experience, the actual chance can be estimated as 1% or even lower. This means a player has to open around 100 cases so as to eventually get a rare item.
To open a case you need a key that costs $2.49, hence opening a hundred of cases will require a pretty penny. Since the chance of a drop is low, the number of “produced” knives is several times fewer than of regular skins. Thus, they are expensive.