This is also introducing the difference between zoom lenses and prime lenses. You can get a good 28mm lens for much less cost than a good 24-70mm zoom lens. Most novices in photography don't start nowadays with good prime lenses, but with cheap zoom lenses.
Great! Inexpensive zoom lenses are getting better all the time. And manufacturing processes are likely also improving. The gap is narrowing.
But, at least today, you still get enhanced features on the more expensive zoom lens, such as wider aperture, and a constant maximum aperture across the entire zoom range. Neither of those things necessarily yields a superior photograph -- you don't need f2.8 across the whole zoom range if you're taking pictures at f6 -- but they can be very helpful. If they're worth paying for depends on one's personal needs, desires, and budget.
>A $300 lens is objectively better than a $5 smartphone lens.
Not sure where you are getting the $5 figure from. In any case, smartphone lenses are manufactured in vastly higher quantities than lenses for interchangeable lens cameras, so it doesn't make sense to compare the per unit cost. Modern smartphone lenses are miracles of optical engineering. See e.g. https://news.ycombinator.com/item?id=30557578 The cost of the R&D that's gone into enabling their design and manufacture probably couldn't have been recuperated if they were being used only in cameras.
Fair enough; perhaps it’s fair to say that given a specific application or lens type, a more expensive one will generally be better than a cheaper one. For example, you can get any prime or zoom you want from Canon as a normal or L variety. The latter will cost about 10x as much, and will be better. 10x better is subjective.
On the flip side, my favorite macro was a Sigma 105mm prime. Tack-sharp, and cost well under $1000. Of course, I’ve never shot with the equivalent Canon L (which isn’t quite the same at 100mm, but close).
A $300 lens is objectively better than a $5 smartphone lens.
A $2000 lens may be objectively better than a $300 but it depends what you're standing in front of and you.
The Nikkor Z 28mm f/2.8 is my favourite so far and it wasn't exactly expensive.
The priority order for things is of course:
1. What the photographer is standing in front of
2. The photographer
3. The lens
4. The camera