What will we glean from your Neptune array that would justify the massive cost? Won’t it just look like this but sharper images of galaxies and earlier and earlier in time? What are we trying to discover then? Surely almost everything about the universe that we can know from our location will be known then. Your Neptune array sounds like one of those bird watchers with the huge telephoto lens just hoping to capture a great shot of yet another bird the world has seen millions of times.
“All science is either physics or stamp collecting.”
Ultralarge telescopes would let you directly image surface features of exoplanets.
There's plenty of stuff we'd like to look at outside our solar system. We could send interstellar probes to local systems, but sending probes to stars thousands of light years away is still going to be pretty tough, even with far future technology.
The hypothetical question to a hypothetical situation deserves a hypothetical answer.
There would be no massive cost. It would be a project affordable by the curious humans/robots/organizations of The Future. Automation will construction and manage the array, and the civilization building such a structure will be post-scarcity to some extent.
At a certain size it becomes possible to directly image exoplanets in enough detail to tell us what the weather looks like, atmospheric composition, perhaps even images of extraterrestrial cities.
“All science is either physics or stamp collecting.”