What is the value of 1851 us dollar coin?

If it's a genuine 1851 Seated Liberty silver dollar, which was the lowest mintage year for that coin, it's easily worth $5,000 in extremely worn condition, and significantly higher in better condition.

If your 1851 dollar has an Indian head on the front and says "1 DOLLAR 1851" on the back, then what you have is a Chinese-made novelty coin that's not worth anything