They post a line (9') for the total, and another for the game.
You can bet into either one.
The total line pretty much starts out at even money, and the book will change the price, as the game goes on.
There were 4 runs scored early. so the price on the "over" went up.
The price will go up and down, like the stock market, as the game goes on. At the end of the game, a "contract" is worth $100 or $0
So, when I was looking at it, the "under" was priced at $19/contract. Now, if you buy it at this price, and the total stays under 9', each contract is worth $100.