I don’t think you can easily just get the sum alone to appear in another project.
Other ideas as workarounds:
You could get the sum to appear in a Universal Dashboard chart (if on Advanced or above plan, I believe).
Or you could “roll up” the sum into a portfolio that the project is contained in.
You could multi-home (add to project) the set of tasks to be summed into a section in the other project such that the sum, along with the tasks, would appear there as well.
