Cross Sec area of earth= 1.3e15
Proportion needed to cover for 1C temp, 1.3%
Assume aluminium foil is used.
Assume that it needs to have 500nm thickness to block light.
Assume most of the mass is ultrathin foil.
So 8.5e6 cubic meters of foil
At 2700 kg/m^3 thats 2.3e10 kg
Making 2.3e11$ at that price tag.
Ie 230 Billion $.
Plus another 41 billion $ for aluminium at 1.8$/kg current prices.
There would also almost certainly be some on-going costs. The Lagrange points aren’t fully stable, so eventually we’d need to ship up propellant for station keeping. [EDIT]: We could likely use solar radiation for station keeping… Probably on much longer timescales mirrors would need to be replaced. These costs would probably be less than 1% of the upfront investment, but if the system gradually falls apart, you then find yourself in a bad situation.
For context, the US congress passed the CARES act quite quickly (relative to normal timelines for passing legislature) earlier this year and that dropped $2.2 trillion into the economy.