My theory is that its value was a mistake. The Eyeless Skull has a nominal value of 30, so FBG added +5 for your effort without checking that the Eyeless skull is actually worth 60+. They've made the same mistake in the Starved War, when you could claim Eyeless Skulls for 30 Echoes worth of wartime contribution. If this skull was worth 70, it'd be a fair but useful Bone Market ingredient.