1500000
2,750,000 elixir, but you get witches.
It is really up to you. If you want your dragons to become more powerful, 20 more damage per second and 200 more hitpoints, then upgrade the dragon. If you want to be able to train P.E.K.K.A's then upgrade Barracks to level 10. But you could actually do both at the same time. While upgrading barracks to level 10 (2 million elixir), upgrade your dragon to level 2 in the laboratory (2 million elixir).
The price goes up some per level. The first upgrade is 10,000 dark elixir. By the time you reach level 40, it will be 190,000.
750.000 elixir and takes 1 day
You need to have the Dark Barracks in order to be able to train Hog Riders and the Dark Barracks are unlocked once your Town Hall reaches Level 7 however you also require the use of the Dark Elixir Storage which you will need to purchase for 600000 elixir in order to build and the Dark Elixir Drill which is the Dark Elixir Collector that you will also need to purchase but for 1000000 elixir.
A level 10 wall cost a total of 3 million. If you want to upgrade your wall from say level 4 to level 10 ,it would cost a total of 5.45 million coins
Raid other people's bases.
2.25million elixir
Barbarians, Archers, Goblins, and Giants
15 laks it upgrade and 30000 to make it
Win your raids.