In Rocket League, sometimes cosmetics can be extremely rare. One example is the White Hat topper. It was only rewards for those who reported game-breaking glitches. Now today it is not rewarded anymore and is worth around $7,000, the most expensive in the entire game.
It's the limited availability that gives this item its worth. There is another item that is worth a lot as well. This item is uncommon and can be found in uncommon drops. The only thing is that it isn't found very often. This item is the Pink Macaron
Pink Macaron Rocket League Explained
In Rocket League, there are several different macarons, each a different color. Years ago, Rocket League and the players noticed that the Pink Macaron looked very similar to the white one. So they decided to discontinue it, and stop putting it in drops.
This discontinuation caused the antenna to grow in price due to the increase in rarity. The item is apparently worth $7, the second most out of all antennas.
Today the item is only available through trades. These trades will be difficult to conduct since the cosmetic item is so hard to find. The price of credits or items needed in return will not be cheap.