Terraform cost for Radar Tower goes way over estimate around 30 seconds in. Is already approximately the height set by the build command yet continues seemingly endlessly until I intervene. Possible bug?

Your terraform went over budget because raising the corner here required a large foundation down in the water. It is unfortunate, but it isn't a bug.

This can be avoided by pressing Space to toggle the mode into 'Cull Cliffs' (provided you're using raise). This cancels parts of the terraform once they experience a blowout in required foundation height. Perhaps it should be turned into the default behaviour.

Edit: Hm, I can see from the shot that you're using level, probably as part of the structure height UI. Level doesn't have the ability to cull large foundations, and doing so could make the resulting terraform insufficiently flat.