I think the JWST is managed as part of NASA's Space Science Directorate. That directorate gets a little less than $8B of the agency's roughly $23B budget. You'd

I think the JWST is managed as part of NASA's Space Science Directorate. That directorate gets a little less than $8B of the agency's roughly $23B budget. You'd have to look at the breakdown by year, but 10-15% of the annual directorate budget is substantial but not absurd given the project.