Jack should have expected the price to be more expensive than the previous rank. He could only exchange for one thing with his available knowledge points.
He could, of course, do more of the lower-level stages to accumulate the knowledge points or come here again tomorrow to repeat the stages. This way, he would have enough points to get the mega spell, but he had other plans for tomorrow and the days after. He also wanted to use the remainder of today to do the challenges in the League of Champions. Otherwise, he wouldn't be able to do it for some time.
Hence, he chose the wing tool. He might have the gold dragon wings which was way better but he also had the Double Clone spell. All this time, one of his clones had to either use the Soar spell, ride on Therras, or stay on the ground. It couldn't contribute much to air-borne battles. With this wing tool, he could have both clones join him in the air.