Proper multiplayer requires quite a bit more money mainly because we would need to bring on network programmers and have a dedicated online QA staff. Also, the total programming dev time would probably increase by around 33% which is quite a bit.
This sounds like complete overkill to me for the kind of MP that the IE games had. "Network programmers" in plural and a dedicated online QA staff? I don't understand how dev time would rise by 33%, that sounds absurd - but maybe I'm underestimating what Roby means with "proper multiplayer". I am talking about the very rudimentary MP the IE games had, with no fancy stuff. Maybe I'm also underestimating how difficult it would be to get it running smoothly. But Obsidian have made MP games before, so they should not need to wrestle with the theoretical questions behind constructing an efficient MP solution.
In multiplayer you need to keep multiple client at sync all the time as in real time game it matter if there are even 0.1 delay between what players see. Which is why you need "network programmers" in plural as there are usually quite lot programming work that they had to do to ensure that network latency don't hinder gameplay.
IE games had engine which had developed multiplayer in mind from beginning, even though Unity has all the necessary components to implement multiplayer mode in game it don't offer any ready-made implementations for one, but instead developers need to actually develop one by themselves. So IE game didn't actually have any cheap multiplayer mode that was easy to implement over single player mode, but instead of they had multiplayer mode build-in from start.
Having co-op multiplayer mode also means that you need to check and make sure that all the interactions, conversations, selections and UI components work as intended also in that co-op mode, which means that you need dedicate online staff, who also need to check that lobbies, server (don't matter if dedicate or hosted) - client connections work always as intended.
And many bring up D:OS as example of simple co-op game even though it has engine that was build from scratch just so that they can offer that kind of co-op experience. Meaning that it was not any simple task for them actually do, but instead whole game starting from engine was build that co-op play in mind, which is why it is so good as it is.