All "Resource Hideouts" or "Dens" work in essencially the same way, they protect a certain amount of your stored resrouces from being taken when a planet is attacked.
As described in game:
Basic: A portion of the metal/crystal/deuterium stocks will be stored here safe from plundering. The Den does not expand the overall storage capacity.
Detail: The capacity of the Metal/Crystal/Deuterium Den shows how much metal/crystal/deuterium is protected from plundering. The better the Den is upgraded, the more resources can be protected from attackers. Espionage probes are able to detect the upgrade level of a Den as well as the amount of hidden resources. The storage capacity is not expanded as a result of the Den.
Cost To BuildEdit
|Shielded Metal Den||Underground Crystal Den||Seabed Deuterium Den|
This has not been confirmed but tentatively the upgrade cost Equation seems to be:
Each level of Den also require one "building slot" from your planet's total size.