Official rule text
24.4 When a player places either a PDS or space dock using the Cost is an attribute of some units that is presented on faction sheets “Construction” strategy card, they take that PDS or space dock and unit upgrade technology cards. A unit’s cost determines the from their reinforcements. number of resources a player must spend to produce that unit.
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Source: Twilight Imperium: Fourth Edition - Living Rules Reference: Prophecy of Kings
Expansion: Prophecy of Kings
Version: 2.0
Reference: Rule 24.4