From what Ajax noticed and guessed with his previous traders with the Nightmare Fiends, Human Elemental Emperors and others, the world cores are the pinnacle resources for the cultivators who awakened the Divine Rules.
Even though Ajax didn't know exactly how much each world core would help the cultivators increase the awakening percentage of their Divine Rules, he knew they could use the Smaller World Cores and the Normal World Cores for that purpose, unlike Ajax who could only use the Greater World Cores.
Even though there were other cultivation resources like Giant Elemental Crystals, Rune Enlightenment PIlls and many others; however, World Cores could be used by everyone and their effectiveness was much higher compared to the other resources.
Therefore, Ajax decided to use the World Core to tempt Prince Dakon into trading the treasures he was hiding in his Treasury.