I am posting this in the hope of saving a few people the time and frustration this one brick might cause, specifically, those people not super familiar with the LEGO games.
In every LEGO game up until now, after you beat the game, you unlock a bonus level. This game does not do that, specifically. Instead, beating the game unlocks a new hub area, called The Bonus Room. The Bonus Room contains the final 10 gold bricks (the other 60 are obtained from level objectives. 9 of these 10 gold bricks are obtained for completing objectives in The Bonus Room, however one of them is very specific, and I don't believe the game indicates this in any way.
In order to unlock this gold brick, you must collect one million studs in The Bonus Room with the multipliers TURNED OFF. In the bonus level of every other LEGO game, the multipliers are automatically turned off, and can't be turned on. In this game, you are free to use them, but doing so prevents the gold brick from unlocking.
This was never clear, and gave me a bit of hassle, so I figured I'd try to help some people out.
I do know the first time the game puts you in the bonus room the screen flashes a message saing "collect 1,000,000 gold bricks to earn a gold brick". Not sure if it says that when you come back or not.
I actually had one or more stud multipliers turned on when I got the gold brick. However, you had to collect enough studs to hit one million without multipliers. So if you had just the x4 multiplier on, you would need 4 million studs to hit the goal. And there were still some studs left when I hit the goal. Not a lot but still a few lying around. Remember to use a flying a character (I like Wonder Woman since she can fly and hit items like Batman) to get all the high spots in the room. There were some big hits high up on the door and posters.