The 90K service is a big one for Infinti's. For a rwd G sedan it'll run you right around $1000-$1200. About half of this cost is down to the spark plugs. 90K is your first spark plug replacement service and those little buggers are $30 a piece, and not the easiest to get to either. So thats a good bit of parts and labor right there. Otherwise services on Infinti's are (ideally) every 15K with the 30, 60, and 90 being the bigger ones. Most dealership service departments will break these services up over a few visits if you can't throw down the whole chunk at once. Something like air filters, in cabin filter, and throttle body service with one oil change, then do your diff and trans fluid services on your next oil change, that type of thing. If you stick to the schedule these cars will run forever lol. Clutches are largely determined by how the vehicle was treated before, but I'd say on average 90K-130K is about right for the life of the clutch, which is also not a cheap job btw, ~$1200+ at the dealership to do it.
If you want to break it down by yearly average though, if you figure 15K a year, with 4 oil changes (every 3750 miles recommended) and either a 15K or 30K type service once per year, plan on about $600-$1000 per year, some years will be more than others obviously. This is also taking in to account that you take it to a dealership to have the maintenance done, if you do it yourself its obviously going to be less.
*small bit of advice: If you can handle doing the maintenance yourself, there are several sites online run by Infiniti Dealership parts departments that sell their parts at a fraction of retail price. My dealership runs one too, I can't plug it on here obviously because I'm not registered as a vender, but know that they're out there. Best way for a DIYer to get genuine Infiniti parts