Buying collectible cards
Two ways to buy: pick a specific card at a fixed price, or open a pack and take the draw.
Fixed price
You see the card, the grade and the price. No randomness, no surprises. This is the right route if there is a specific card you want.
Packs
A pack costs less than its top cards are worth and more than its common ones. That gap is the trade you are making, and the odds table tells you exactly what the trade is.