2 Samuel 24:24

24 And the king said unto Araunah, Nay; but I will surely buy it of thee at a price: neither will I offer burnt offerings unto the Lord my God of that which doth cost me nothing. So David bought the threshingfloor and the oxen for fifty shekels of silver.

1 Chronicles 21:25

25 So David gave to Ornan for the place six hundred shekels of gold by weight.

This is one of many times that the wording of a certain event is slightly different between Chronicles and Samuel or Kings, and as a result, these passages get rounded up in lists of alleged contradictions.

So, did David pay 50 shekels of silver or six hundred shekels of gold for the threshingfloor?

Difference in Value

Note that this is a massive difference in money. It would be like comparing $400 to $1,000,000.

That's because 1 Chronicles is recording the price that David paid for the entire area where the threshingfloor was located:

1 Chronicles 21:22

22 Then David said to Ornan, Grant me the place of this threshingfloor, that I may build an altar therein unto the Lord: thou shalt grant it me for the full price: that the plague may be stayed from the people.

The writer of Samuel saw it fit to record the price of the actual threshingfloor itself, which as we learn from 1 Chronicles, was paltry in comparison to the entire price of the land that David actually bought.

Conclusion

As is often the case when comparing mutual events in Samuel, Chronicles, and Kings, the author simply chose to focus on a different aspect of the story when retelling it than another did, according to what they thought was important to highlight based on their perspective. Both are correct.