2 Samuel 15:1

Absalom Leads an Insurrection against David

15:1 Some time later Absalom managed to acquire a chariot and horses, as well as fifty men to serve as his royal guard.

2 Samuel 24:24

24:24 But the king said to Araunah, “No, I insist on buying it from you! I will not offer to the Lord my God burnt sacrifices that cost me nothing.”

So David bought the threshing floor and the oxen for fifty pieces of silver.