Questions about electricity generation cost

[a17321]

I am using GCAM to explore the impact of halting new coal power on the power system. However, the electricity generation cost in the model output makes me confused. For example, if I stop new coal power installation after 2020 in the model, then the electricity generation cost (which the unit is 1975$/GJ) in model output will all be zero after 2020. I wonder what the generation cost (by tech) in the model output represents? And how it is calculated? Many thanks!