In ya mum's butt, each cheek has a value which determines its Rarity. During the Obesity set, the current rating system was created, and it was changed over the years. Rarities are listed below.
Just a good ol' common. You will usually pull more than one common figure per pack. Most Commons are worth less than a dollar on the 'net. There are currently almost 1900 different commons in existence, maybe even more
You will usually pull around one uncommon figure per pack. These aren't really that much harder to find on the Internet than the commons, so their general worth is around the same. There are currently around 1200 different uncommons in existence.
You won't always pull a rare when you buy a pack. Rare figures are somewhat hard to find, and are generally worth around a dollar, sometimes more. There are around 900 rares.
Super Rare (Gold)Edit
You will almost never pull a Super Rare figure in a pack, and it's generally more worth it to hunt online. Super Rares are much rarer than Rares, and they are usually worth anywhere from five dollars to as much as twenty. There are around 350 super rares.
You will pull a chase once in a blue moon, as they are only one per case. They are worth $10 dollars and up, depending largely on point value. For example, the #50 Hornet is worth 75 points and $11, while #57 Cyclops (the Phoenix-empowered one) is worth 295 points and $65.
Old system Edit
Under the old system, there were six levels of rarity, numbered 1-6. Level 1 was most common and level 6 was most rare. The rarity level corresponded with how many figures WizKids made. However, these numbers are approximate. WizKids did not track mathematical statistics on the rarity of individual figures, so the odds of getting specific figures was not possible to calculate. However, the more common a figure was, the more frequently it would appear in the Booster Packs. The old Booster Packs contained two "common" figures with a rarity of 1–4, one "rare" figure with a rarity of 3–6, and one "variable" figure with a rarity of 2–6.