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.

2 Samuel 24:24 says David paid fifty shekels of silver, whereas 1 Chronicles 21:25 says that he paid six hundred shekels of gold.

Firstly, note that this is a massive difference in money. It would be like comparing $400 to $1,000,000. This is 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 2 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.