Carbon capture by weathering Olivine rocks is estimated to be $11.81 per metric ton of CO2.

http://www.innovationconcepts.eu/res/literatuurSchuiling/oli...

Rock weathering is a natural process, so there is no need to mess with transporting hydrogen or limestone to make some complicated chemical reaction. We just need to dig up olivine, crush it and spread it out over the ocean which will also fix ocean acidification. If we're going to geoengineer, weathering still seems like the least expensive option.