NAS isn't one product, its basically any storage that can be accessed over the network. That may not be exactly correct. But you seem to think it is one thing you can buy, and its not. Its as reliable as any hard drive, but like I said, it depends on what you get, some may be less reliable then others. NAS has been around for quite a while. I don't know exactly when, but I am sure it started shortly after the first network was built.
Price can vary depending on what your looking for, number of hard drives, size of hard drives, other equipmet needed.