Yep, exactly this.

The cost of CO2 capture, and conversion into usable fuels, is in the cost of the setup of the infrastructure etc (as well as cost to run the pumps once setup, which in this case is where the free electricity goes).

The return on such an investment is likely negative, because the synthesized fuel does not sell for much (compared to the same fuel that is extracted off the ground and refined - look at natural gas as prime example). Therefore, even if electricity is negative (ala, free), you cannot make money from doing it.

Either the cost of the carbon emissions is captured as part of the cost of fossil fuel extraction (and returned to this carbon capture/conversion system) to make it break even, or something else has to happen (like massive efficiency increase in doing such conversions) in order to make it economical.