Run away from that dealer! You can buy the actual OEM sensor and
install it yourself in under 5 minutes.  You can purchase the
sensor on-line from several Nissan dealerships for around $85.
Check the 5th Gen parts thread at forums.maxima.org for a list
of Nissan parts departments with great on-line prices.

Ignore the noise and keep using 87 octane fuel..

Use octain boost or high test gasoline and see if the problem goes away.
 Also, have you done a tuneup yet?  At 76K my plugs were gone and the
new plugs made a great difference in performance.

From what I've read, MAF failures result in a no-start / no-run
condition.  Also, instead of replacing a $500 MAF, supposedly you can
typically replace just a $50 part within the MAF (sensor?).
Also, many have reported that just cleaning the MAF (very carefully)
has renewed the performance of their cars.  I will defer once again to
www.maxima.org for details.  Beware though, a lot of the posters are
19 year old kids driving daddy's car.

If you are somewhat technical, you might try getting one of the
$100-150 OBD-II program/cable combos for your laptop/palm/etc.  This
will let you read codes and do diagnostics, but it won't necessarily
spell it out for you.  You'd probably need a reference, such as the
factory service manual to compare readings with.  Let me know if you
want some links.

pv