Why did David insist on paying for the threshing floor where he built an altar?

Answer: He'd not offer what cost him nothing

"I will not offer to the LORD my God burnt offerings that cost me nothing." Then the plague on Israel was halted.

“Araunah said to David, “May my lord the king take whatever seems good to him and offer it up. Here are the oxen for a burnt offering and the threshing sledges and ox yokes for the wood.”— 2 Samuel 24:22 (BSB)