Let’s say you need to reflect 1% of the light reaching the earth’s disc. If you’re using solar sail like devices with an average areal density of 10 g/m² and you have launch cost of $10 per kilogram (using LEO figures because devices made really lightweight can actually move themselves around with pressure from sunlight)… about $150 billion in launch costs. Material/spacecraft costs are a big question mark and can vary by 5-6 orders of magnitude, so let’s say it’s $10/kg, for a total of $300 billion.

Still sub optimal compared to other solutions but it’s possible we may need to try something like that.