A CS2 case is a locked container. It holds a set list of weapon skins across rarity grades, and each case has one rare special item slot that can return a knife or gloves. Cases drop to Prime players through the weekly care package and can be bought and sold on the Steam Community Market and on third-party marketplaces.
Opening a case needs a key bought from the in-game store. The key is the real cost, since many cases are cheap. The item you get is random, and the odds are the same for every case, with rarer grades far less likely.
What are the odds of opening a knife from a case?
Valve published case odds for one region, and the community treats them as the global rates. Roughly, Mil-Spec (blue) items appear about 80% of the time, Restricted (purple) about 16%, Classified (pink) about 3%, Covert (red) under 1%, and the rare special item, meaning a knife or gloves, about 0.26%. About one in ten weapon drops is StatTrak.
Those odds mean the average value of what comes out of a case is usually well below the price of the case plus the key. Opening cases is a paid chance, not a way to make money on average.
Why do case prices change?
Case prices depend on how many are still dropping. A case in the active drop pool gets new supply every week, so its price stays low. When Valve moves a case to the rare or discontinued pool, new supply slows or stops, and the price climbs over time as people open the remaining ones.
That pattern is why some traders hold sealed cases as a long-term position. It is also why a new case release, which changes the drop pool, can move the price of older cases within days.