Basically like this.
Make it so you can change elixir into gold and the other way around, but it's a certain ratio.
(Ratio gets closer to 1-1 as you upgrade).
The exchanger can be unlocked at th6. An example chart can be like this:
At level one (TH6)
Elixir to Gold: 2-1
Gold to Elixir: 2-1
Dark Elixir to Gold: No Dark Elixir at th6
Dark Elixir to Elixir: No Dark Elixir at th6
Cost: 10000 Elixir
At level two (TH7)
Elixir to Gold: 1.75-1
Gold to Elixir: 1.75-1
Dark Elixer to Gold: 1-15
Dark Elixir to Elixir: 1-15
Cost: 50000 Elixir
At level three (TH8)
Elixir to Gold: 1.5-1
Gold to Elixir: 1.5-1
Dark Elixir to Gold: 1-20
Dark Elixir to Elixir: 1-20
Cost: 100000 Elixir
At level four (TH9)
Elixir to Gold: 1.25-1
Gold to Elixir: 1.25-1
Dark Elixir to Gold: 1-25
Dark Elixir to Elixir: 1-25
Cost: 500000 Elixir
At level five (TH10)
Elixir to Gold: 1-1
Gold to Elixir: 1-1
Dark Elixir to Gold: 1-30
Dark Elixir to Elixir: 1-30
Cost: 1000000 Elixir
