As a point of reference, the Galileo navigation constellation apparently costs about 10bn (€ not $, but call it even): https://qz.com/1264365/brexit-is-breaking-up-galileo-europes...
That system is essentially a modern re-implementation of GPS, a known, proven technology.
To suggest that this untested, unproven, highly speculative solar shade project would cost the same is laughable to me. You're also totally discounting the unintended consequences and second-order effects of trying to engineer the climate.