All units have costs related to purchasing them. This cost will always include at least one resource and may include sacrificing one or multiple units. This is often one of the more common uses of resources during games, due to these costs generally being higher and having more lasting impact.
Unit costs are often listed by giving the number of gold required followed by one letter (R, G, B, E) for each required resource. For example Centurion's cost can be typed as 18GGBBR. In this Wiki resource symbols are used instead (e.g. 18).
Unit abilities may have costs related to them. This cost may include resources, sacrificing units or losing health in the case of Fragile units. Generally, only units with Blocker will have abilities without costs associated with them, as the inability to block after being clicked it itself a cost.