This paper estimates the cost at ~$2.25B/year, and notes that more than 50 countries have military budgets exceeding $3B: https://iopscience.iop.org/article/10.1088/1748-9326/aae98d#...

They point out the plan assumes you can launch planes from a few different points around the globe. You don’t need global agreement, but at least access to some foreign airports.