I have the feeling the cost shouldn't be in $ but in energy used, right?

Because at the bottom line this is what really matters - we release CO2 to the air because it's getting us energy, so what would be the energy cost of capturing back?

Maybe you think it doesn't matter because we're capturing CO2 with renewable energy, but renewable energy isn't completely carbon-free (when you take into account creating and shipping the equipment) and that renewable energy could have been used to replace carbon generating energy.