Nexus = new Citadel?

Ark to Nexus scale looks like Sovereign to Citadel. Now you can say that we weren't given size of Arks and be right about it, but now I have an impression that they are even bigger than Sovy.

The scale of the AI is just absurd handwaving. It would take the dedicated resources of all the advanced races to pull off such a venture in "reality". I understand why they didn't want to, but not tying it to the Reaper invasion is just silly. "Someone" is being boneheaded and stubborn. To think that a few wealthy people would invest the entirety of their personal and corporate wealth into a venture that at best would see a payout hundreds of years after they've turned to dust is beyond believable. You might encounter one or two people with that kind of vision, but by and large humans (and reasonable to assume the aliens too) are far too shortsighted for such a venture. It's a shame we have to start the game with immersion shattered like that.

It'd have been the same if it was tied to the Reaper invasion though, because while it'd help in making more believable that people started that project (and yes, all the 4 council leading species are in more or less in the same way, I think), on the other hand it'd shorten the time for the whole project to be operative to at least half, if not more (the vast majority didn't believe the reaper threat was real until ME3).
There wasn't a perfect solution in my opinion.

I just need somewhere to get drunk and dance.

On ashes of Kett (or Khet? still not sure GI wrote them right) world?

thing is besides a mass effect tooth brush no one really knows the prices of anything in ME , and it's probably a hyped overpriced gadget.
So how much does the Nexus cost really? We're not in 2016 here . 200 years ago, paper and ink were something to use carefully
as those items were very expensive. But labor was cheap. Oil was known but rare and no one cared about it, etc.
So who can say how the economy works in 2183 or whenever.

Bill Gates ( since he's been mentioned) is donating his fortune to charity , alongside Buffet, and probably others we don't know. Heck i knew a millionaire who gave his fortune to Salvation Army 10 years ago,( why not me i ask you ) .
So as far fetched as it may seem, is it entirely "impossible" for that project to be run by some NPO that amassed all that was needed ?
Shepard spent his entire life doing the absolute impossible, yet he did and we all went along, also accepting a bunch of incoherent details and ridiculous assumptions within the Trilogy ( like XXth century pop music everywhere on the Citadel, in clubs, in parties , in Shep's bedroom. That's like going to Osheaga to only hear Bach and Handel ).

/edit

as for the technology used, i love and embrace space magic ! 

Private companies decide to invest enough money to put galactic governments to shame into a project which may or may not yield something in 600+ years.
That's long term investment if I've ever seen one.

I think that the Nexus will be the temporary headquarters/government building of the AI and I believe that if that is the case then the Nexus will leave for the Andromeda Galaxy well before the 4 Ark ships ever leave to get everything set up

I'm pretty certain Jien (can't recall the last name) did say that the Nexus would be departing first.

I think this is exactly the long term goal, at least as implied in the video orientation. Certainly things will become more exciting for the expeditionary team than anyone hoped.

Long term, if the series continues long enough, I suspect we will see a bridge of some sort connecting Andromeda and the Milky Way. It could be via a planned Mass Relay, or it could be involve Remnant tech with which we are as yet unfamiliar. Maybe it will involve something else entirely. Whatever the case, I'm confident it will happen, and we'll then have two (mostly unexplored) galaxies. The potential for meeting, merging and clashing of civilizations in such a case could make for good stories; but I'm getting way ahead of the present.