In preparation of the upcoming Clan Wars, Supercell has discounted the cost of boosting resources. For a limited time, boosting any resource collector — be it gold, elixir, or dark elixir — will now cost just one gem each. The boost lasts for 24 hours.
Supercell says discounted boosting will last until the Clan Wars update is live. I'm not sure when that is exactly, but with an overview of the new feature coming soon, I imagine it won't be long. Buff those defenses while you can!
We'll be sure to keep you updated with anything we learn about Clan Wars in the coming days.