I've never heard of a brake fluid flush being required by warranty. If that's the case, then everything in the Maintenance Guide would be a requirement. I can see a warranty claim being denied if the car was neglected for so long that it caused a problem.
The only thing I know of that you MUST do to keep the warranty in effect is the annual battery check.
I was quoted $200 by a Nissan dealer when I asked for a quote on a 1 year service. It included the battery check (supposed to be free), all the inspections, tire rotations, cabin filter replacement, and brake fluid flush. That's the "severe service" recommendation; the normal service one omits the brake fluid flush. I just told them to do the battery check and paid them $20 for a tire rotation; most of the inspections are covered under Nissan's complimentary "safety check."
But at the 2 year mark, these services are recommended for even the normal service schedule, and I plan on spending the money at that time. So this is my breakdown of what I expect my Leaf to cost me in servicing over my 3 year lease:
6 months: $20 for tire rotation
1 year: $20 for tire rotation, free required battery check
18 months: $20 for tire rotation
2 years: $200 to include free battery check, tire rotation, brake fluid flush, cabin filter replacement, inspections
30 months: $20 for tire rotation
36 months: give car back to Nissan
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Total expected servicing costs over 3 years: $280
That does not include tire replacements, which based on others' experienced will be likely before my lease ends, nor any unexpected events. Also, if you frequently drive on rough or dirt roads like at least one member here does, I would recommend going by Nissan's severe service schedule which will also recommend yearly brake fluid flushes and inspections every 6 months.