You are likely aware that 24-80 is an odd range for an APS sized sensor
camera.
The equivalent range in 35mm terms is the 17-55. The newer Nikon VR version
has been favorably reviewed, however I believe the Nikon 18-70 is still
better optically.
For the uses to which most of us put these lenses the cost of name brand
glass, as well as measurable performance of most lenses, is not rationally
justified. In many cases Sigma/Tamron lenses outperform camera brand lenses
or have cost advantages that reasonably outweigh any technical differences.
Realize also that many lens reviews do not translate to real world
experience. Having had frequent opportunity to use camera brand and other
lenses I can only assume that reviewers are often being handed cherry-picked
lenses that have been optimally tuned because the lens you buy off the shelf
is not.
In any event most of us are far more burdened by our aesthetic limitations
than the technical quality of our cameras and lenses.
You have only to follow the links when brave posters here show off their
images to see that is the case.
Get a lens that you are financially comfortable with and enjoy using it.

    Just point your browser to web page like www.fredmiranda.com and read as
much and as many end users' feedbacks as you wish.  And I believe after
hour(s) to go through lot of choices and opinions you should have clear idea
what you want or what you may need.

My wife and I have the Sigma 18-50mm f/2.8, which is an exact equivalent of a
full-frame 28-80. It's a bit bigger and heavier than you might expect, but it
does a good job. And it maintains its full aperture over its entire zoom
range. We have the version with the Canon mount, but they make it with a Nikon
mount as well. I think it's less than a third of the price of the Nikon lens
to which you refer.

Bob