Three Interstellar parts (ISRU Nuclear Fuel Processor, Antimatter Catalyzed Fusion Reactor, and IXS Antimatter Collector) currently have a zero cost in-game. This results in a negative cost when their resource containers are empty, in the cases of the ISRU and the antimatter collector.

The costs initially listed when you've placed the part change oddly when you make any change to the part's settings--in the case of the ISRU Nuclear Fuel Processor, its 'cost' when initially placed is -83,500v. When any of the (initially empty) resource bins are added to, regardless of emptying afterward, that 'base' cost suddenly jumps to -120,375v, and if all bins are filled, the cost is 0.

Tested on Kerbal Space Program 1.1.11 and .12 with KSPI-E and the following mods, all installed via CKAN:

• Interstellar Fuel Switch (Required)
• TweakScale (Required)
• Module Manager (Required)
• KSP Recall (Required)
• Community Resource Pack
• Community Tech Tree
• Hide Empty Tech Tree Nodes