I'm only answering to sum up to combine the 2 answers, and add drill percentages
Read Mathias711's answer for available storage loot, available collector loot is 50%, for dark elixir drills, it's 75%. This is confirmed here
"The percentage of Gold and Elixir that can be stolen from mines and collectors is 50% and is capped only by the storage capacity of the resource collector."
"The percentage of Dark Elixir that can be stolen from drills is 75% and is capped only by the storage capacity of the drill."