$3-5B would go a long way to a new Einstein telescope (ie. gravitational waves detector). It would likey allow for significant sensitivity increases in the existing ones. It could propel the National Ignition Facility or ITER forward. It could be used to build a new Arecibo. It could add an enormous amount of low-frequency telescopes - or other scopes for very large integrated scope arrays.

I'd much rather broaden our view than double it in one narrow band. (Or advance fusion research.)