By comparison, direct air capture of CO2 costs something like $600/ton. At 40 billion tons of global CO2 emitted per year it could cost 24 trillion per year to remove current emissions.

That said, I would bet there’s better economies of scale / ability to innovate lower cost zero-carbon energy sources with direct air capture rather than a giant space project.

My napkin math is that a bubble with a shadow the size of Brazil and made out of graphene weighing 0.77 mg per square meter would weigh about 25 million kg. Using multiple smaller bubbles could reduce this by at most a factor of four.

At a cost of $1,000-$10,000 per kg it seems possible to get this to space with a total cost on the order of hundreds of billions of dollars.

You could do a estimate of the graphene itself but I think the cost would be similar.