The Skull costs 10000 Coins. It used to be the most expensive item in game, along with the Angry Brai voice pack. After the newest gear update, it is now the 3rd most expensive item. This hat makes you look like a skeleton. It is just for decoration and it does not change your stats or any specials. The skull has gone through a few iterations. It used to be smaller and sometimes the hair would show through, and as you can see in the GIF, weapons pass through the skull instead of going behind it.
Owning a skeleton hat shows that they have a lot of money, or they are one of the elite/veteran players of the game. Tons of prestige go along with the hat.