Mystery Boxes offer you the chance to win one of several prizes, including Prime and Legendary players.

Mystery Boxes are purchasable with gold. When you click on the "I" [for information] to the bottom right of Mystery Box, you will see the list of potential prizes which may be won.

To see the odds (OUT OF 1000) of winning each of the potential prizes, either tap the % sign, or the text that says "View The Chances For Rewards." on the info screen:

All mystery boxes contain some kind of loot. Your reward is determined randomly, with some items having a statistically greater chance of appearing [weighted odds] than others: anything from, say, a stack of blue bucks (most likely) to a rare premium item like a Prime or Legendary player. (much tougher odds.) The odds are out of one thousand.

The picture above shows the top prize: 10/1000 chance of getting a Max 400 Prime Player., a very rare and valuable prize! (As you may remember from your high school math, this is NOT the same as a 1% chance, that's not how probability works!)

If you win a player from a Mystery Box that you already own, your existing player will be leveled up, unless he’s already capped at his division. In that case, you will receive XP.

For most boxes, your odds of any given reward are the same each time you open; that is, the odds are not affected by previous boxes opened. "Guarantee" boxes are an exception:

Occasionally you may see special "Guarantee Boxes" which will guarantee a high-value type of reward after a certain number of boxes are opened. When you first buy one of these special boxes, a counter is set. Let's say the number of boxes is ten. Once you buy your tenth box, you will definitely receive the guaranteed type of prize. You may, however, receive that guaranteed type of reward earlier, say, on your third try. In that case, the counter resets to zero, and you need not purchase any more to get the prize, unless you want to try again for the high-value prize offered.