I also paid around £10 from NissanI think I paid ~£10 for mine (not from Nissan). - Nissan have to have a supplier too, so I'd suggest if you've heard of the brand (like UFI) it will meet OEM quality.
I'm not sure what the "upgrade" filter would be, or if there is one; the GTi-R already has a filter that's bigger than the Silvia, and might be the same as the Skyline. - Seems to me that there's plenty of headroom in that.