RHB page 55 Storage Compartment 1st paragraph second sentence states "Each Slot dedicated to storage can hold approximately two kilograms or two litres and costs Cr50." The airlock portion is not included in the base storage compartment.
Here is the entire write-up so that nothing is taken out of context.
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A storage compartment is an internal space within the robot sealed by a door or panel, usually accessible to its manipulators. Each Slot dedicated to storage can hold approximately two kilograms or two litres and costs Cr50. The storage compartment is protected by the robot’s armour and environmental enhancements but does not include an airlock.
Hazardous material storage requires special materials and other safeguards and costs r500 per Slot. In general, the material to be stored should be specified upon installation of a hazardous material storage compartment. If an airlock is desired, it should be considered as a separate hazardous material storage compartment of a Slot size large enough to accommodate the item that requires it."