As pointed out, there are a few 'low cost' options; biochar, plants, algae, etc but they are still extremely expensive since the opportunity cost is essentially

As pointed out, there are a few 'low cost' options; biochar, plants, algae, etc but they are still extremely expensive since the opportunity cost is essentially 0. The ability to fix nitrogen from the air and convert it into liquid or solid form on an industrial scale was one of the most important breakthroughs in human history. We can do the same with CO2 but it's cheaper to dig it out if the ground and there is a lot less of it in the air. N makes up 78% of air by volume. CO2 is only 0.04%. You need to handle a lot more air to get the equivalent amount of carbon as you do nitrogen.