Rule 356.1.b.3

Further additional costs and/or cost increases applied in subsequent steps may raise the card's Total Cost above zero. Example: Legion Rearguard is a Fury unit that costs 2 Energy and 0 Power and has Accelerate. A player plays Legion Rearguard and is instructed to ignore its costs, but chooses to pay the Accelerate cost. They ignore Legion Rearguard's Base Cost of 2 Energy, but the optional additional cost of 1 Energy and 1 Fury Power is added to its Total Cost and must be paid.