Time needs a lot of quanta anyway, with hourglasses and pharaohs and other high costs like eternity. So a dragon costing 12 time isn't actually as big a deal to pay for as it would be for, say, earth, where there are usually less towers in a mono deck.
For the entropy/darkness difference, darkness has pests, steal, drain life, eclipse, dusk mantle... Lots more powerful cards and ways to make the dragon benefit. While entropy has antimatter and discord and that's almost the extent of it. So entropy's dragon costs less because it's from a less versatile element.
Air needs more attackers, and its dragon has more benefits compared to the darkness dragon for the same reason as entropy. Air has eagle eyes, shockwaves, and unstable gas. What else could a mono air use to good effect? Nothing, so its dragon is a little bit stroner.