I never thought of it that way. I just took it as a sign of excitement being in the rich country and meeting his cousin after leaving the misery of his old life and being stuck on a boat for weeks.
I mean I also found it a bit odd, as it sounds almost as an attempt for an insult kinda - but he's with Roman, who is known to be easy going so they are both cool to each other. When Niko is being sarcastic, it's always quite obvious.
Regardless, I think it's quite overthinking it, even though admittedly so have I.
And Niko is obviously surprised when they actually reach Roman's flat so it's not what he expected.
I don't really think he's too surprised, but I think he was very pissed that Roman managed to make him spend 7 months travelling half of the world to discover that Roman was a liar. Roman tells a bit of his and other Eastern European immigrants' realities on the way to the appartment, and Niko drove around the place, so I don't think it was such a shock.
I have to disagree on that. Niko actually did seem visibly surprised as could be clearly seen from his remark, "THIS is the mansion?" The part where Niko doesn't appear too surprised once inside the house was due to Roman's remark "The mansion is coming cousin, that's the dream", to Niko's disbelief. What may have started out as just a suspicion on the docks about Roman's so called american dream, was being very carefully being opened in front of Niko's eyes on the way to the house itself. This is evident by Roman's remarks such as, "We are passing the cab office Niko, this is the business that will take us on the top of the pile". Why would this business take them to the top of the pile if Roman was apparently already living the american dream? Niko's suspicion that was only triggered a little at the docks, seeing the cab was now confirmed when they reached Roman's place. The notion that Niko was already aware of Roman's condition and was being sarcastic is not very plausible since we see Niko, fresh off the boat, he meets his Cousin after a very very long time and only had been hearing a lot about him and his so called dream life.
It is very unlikely that he was aware of the ground reality seeing as Niko was being as excited off the boat with Roman as he was on it with Hossan, describing the latter of the wonderful life his cousin Roman apparently leads. So Roger's statement about the excitement of arriving in a new country and meeting his "fancy" cousin is very valid. It's pretty evident just by the dialogues exchanged by the two down at the docks. "I had quite the last night, two women...the land of opportunity, I've made it!" "My cousin is here, we are going to rule the world", depicting Roman's belief that he could use Niko's help to actually live the american dream, while Niko could have easily perceived this as his contribution to the already flourishing business enterprise he apparently thought Roman was running. Also, if you read the original transcript and even without doing so, Niko's statement is "Roman, come on, let's go... to the mansion, eh?". Not uhh huh. He was not trying to be sarcastic but this notion was kept to keep the audience's curiosity on the up, making some of us suspicious of Roman's claim right then and there while for the others, just like Niko the suspicion was confirmed once they reach his place.
If I had to point out one thing that still doesn't quite fit from that night is, After they enter Roman's little "apartment", we see a visibly disappointed Niko which I expected to last for a little longer but after Roman's joke about how he has the "best cockroaches, the best dirt", we see them sharing a laugh and taking it as a light situation, as evident by their conversation that follows.
Now given that as we progress, we can hear Niko's remarks that are still filled with disappointment, however I really don't feel that the entire scenario after Roman's one joke was kind of making sense, but again maybe it was Niko's quick thoughts and an already un-confirmed sub-conscious suspicion on Roman's claims down at the docks that made him feel that he kind of expected this and atleast they had a place to live and the whole situation could have been a lot worse.