I have a 2015 Nismo.
Mods: Stillen Gen 3 intakes - $600, eBay test pipes - $115, Top Speed Pro-1 axle back exhaust - $300, ACT Lightweight Steel Flywheel (17.4lbs) - $400 (though I'm not sure this attributed to any power gains)
Finally I got my car dyno tuned at Dynosty. I baselined at 311whp on a Dynojet dyno before the tune. With my Ecutek tune I ended up at 325whp. The total cost of the tune including dyno time, tuning license, and tuner cable-which you don't need to purchase if you're getting it dyno tuned-was around $1,000. It would have been $300 cheaper had I opted to not buy the cable.
I've read stock Nismos usually dyno anywhere between 290-300whp. So for around $2k, give or take a few hundred dollars, you can add over 30whp with ease. Without a tune you could easily get over 300whp for around $1k. I hope this helps.
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