Verse 16 already said this!
Radak: Before, the verses were concise. Now they elaborate to explain the dialogue, and how Elisha's words were fulfilled.
Malbim: Before it discussed the price in Machaneh Aram, outside the city. Now it discusses the price in the market in the city. (a) Normally, the city price is higher, due to the king's tax. (b) Had the Shelish not been trampled, the spoils would have gone to the king's storehouse, and the Shelish would have raised the price, to falsify the prophecy 1 .
If the king insisted that all the spoils belong to his storehouse, would he not also fix a high price for his own profit? Before, people paid high prices for a donkey's head or dove excrement (6:25). If no one could keep spoils for himself (even if he ate to satiation in the camp), the king could still get a high price for grain! (PF) Also refer to 7:16:1:1*.