Im not too worried about the cost, its more the utilizing existing tech to achieve something and not just inventing something from nowhere. If we are getting new tech then id like a reasonable explaination as to where it came from.
I cannot really get with the sudden need for having any new technologies be given a "reasonable" explanation, mostly because every single game has "miracle " technological advances that were given no explanation at all used as the plot movers for the hero's journey
In ME1 you have the cypher beacon: A Prothean technology that can alter organics minds to understand Prothean communications in a way that millenia of study by a race intimately linked with Protheans (The Asari) could not achieve.
In ME2 you have project Lazarus: A technology that can somehow reanimate the dead "perfectly"
In ME3 you have the Crucible: a device that has one of the most implausible design evolutions in Sci-Fi, a device that is derived from an unknown number of extinct races that did not co-exist in time somehow communicating their most advanced solutions to an unsolvable problem that they had no foreknowledge of (if the Reaper MO of the Prothean Invasion is true of previous cycles)
Probably most unbelievable though happens off screen: the human discovery of the MASS EFFECT. Lore states that we discovered the Mars archive in 2148. It also states that we were able to translate an understanding of Alien data that is of an evolution state that is far beyond our own, and uses a material and science theory that was totally new to us. Someohow we developed an understanding that pointed towards the Relay. This was discovered in 2149, entombed in 100's of kilometres of ice. Somehow we melted this down (The volume of Charon is roughly 9.32 * 108 km3), in temperatures at around -2200C in a matter of months, Switch on an inactive relay and we also had developed ME capable vehicle technologies and had constructed an unknown number of probes, and then human vehicle. All that in a window of at max 24 months?
Did BioWare ever give any form of explanation to these in their lore or games, let alone a "reasonable" one? I don't think they did and, while critical comment on ME2 and ME3's "miracles" was pretty vocal, I did not see much criticism on ME1's "miracle" and even less at the "miracle" that the brand name itself is built from until after ME3.
Mass Effect was built from the Combination tropes of "Technological Uplift" through a sudden import of "Alien Phlebotonium". Humans, Asari, Batarians are noted in lore as being "uplifted" by Protheans. Cerberus being empowered by Reaper Tech in a similar fashion and the Crucible derived from an unknown series of tech uplifts and unknown number.
That will probably be the root tropes used to justify the ARK and the technological uplifts already in the series are possibly even more impossible to imagine than an ARK. Current human speculation of Intergalactic travel offers several concepts, but we have no concepts that speculate that we could translate an alien language and technology at first contact in the manner introduced with the Mass Effect franchise and brand name, we have no speculative theories on how to reanimate a corpse back to life, let alone reproduce the intellect and "soul" of a human.