I don't think it's a matter of cost here. I think it's NYC and it's a matter of storage, literally. 500 square foot apartment is fairly common. Car seats take up a TON of space, truly.
So, with your inlaws having one, I would say just go with that, personally. How far away are they? If they're walking distance, then it's definitely fine. If you find you need another because you're doing more trips, you can stop by a babies r us or a target or a kmart pretty easily from any of the five boroughs or online.
You will likely just take cabs less when the child is in tow, in my experience. Pedatricians here can't be just shown up at, honestly. Even very responsive ones, if it's not ER-worthy, you're going to have 2-8 hours before even a same day appointment. You can plan to subway, walk, or get in-laws to drive you by then.
I live in NYC, two kids, and I now have 2 carseats--radian 65 and ride safer travel vest. The radian is $$$ and god-awful heavy, but I don't have in-laws with carseats and we use zipcars somewhat often. It works from very little (not ideal for newborns, but apparently it can be done) to 65 pounds. It folds up, meaning that it fits under the crib/toddler bed, which was a big plus for us. You're not going to carry it around "in case" of cabs though. As I said, it's HEAVY. When the kid is bigger, the travel vest rocks. It's for 30 lbs and up I think, basically a vest version of a booster seat. I have carried it around for just in case.
The only times we had to go to an ER or urgent care, we walked. So do consider that as well. How far are you from a hospital? And it's not especially likely that you'll have to go there, from home, and need a carseat. They are used to parents not having them in NYC I have to assume. Actually, the one family I know who went in an ambulance, they had something for the small child to ride in.
I would say no, you don't need a car seat. I would not go buy a big seat like the Scenara, personally. I'd buy something that folds (the two radians are the only ones I know that do) if you feel you need one personally and I'm correct that the $ is not the issue, the storing of a big, weirdly shaped object is. And they work to 65 or 80 pounds depending on the model, so I imagine it's pretty much for the entire carseat/booster seat age.
Edited by EviesMom - 7/13/11 at 11:35pm