The main advantage of nitrogen is more consistent pressure (doesn't change that much pressure due to temperature), this is particularly important for racing drivers. I have tried once, couldn't tell the difference in real life (just like RON95 vs RON97, but that's another topic), so I continue to pump with normal air, and I also get to experiment with different tyre pressure. It is not financially viable to maintain using nitrogen IMO.
RM30 for 4 tyres. Not sure how much to just top up... I guess I'll find out when the next service is due.
Main reason: they don't have the exact same model of tyre that I had. My area here FOC when you buy tyres from them, so each time if lack nitrogen, just have it refilled. I find it very lasting compare to using normal air :/.. though it pretty much depends on the quality the shop uses i guess. The one nearest to my place always last me very consistently (never had a refill for more than 6 months).
