Interesting. 28’s actually tend to sell at a premium. They are considered the perfect “bird” (quail) gun across most of the South. They are built in smaller numbers, and in a quality SxS or OU are aimed at a presumedly more discriminating clientele.
4x to 6x the price of 12 bores. (in mid-grades or modern guns)
Since we're talking about high quality guns, i'll make broadbrush statements that relate to British guns. (I don't play around with best Italians)
In the history of the British gun trade, 96% of shotguns were 12 bore.
About 1.5% 10 bore.
About 2% 16 bore.
All the other gauges: 4 bore, 6 bore, 8 bore, 14 bore, 20 bore, 24 bore, 28 bore, 32 bore, 9mm, and .410 make up the residual.
Thus, people go crazy about smallbores because they are the micro-remnant of all guns ever produced. Subtract guns purpose built for the aristocracy's women and children, "grown up man" small bores are pretty darned rare in vintage British guns.
Two other rules that are helpful to our discussion the readers should know:
Rule 1: Greener's Rule of 96. The proof mark load on the gun, multiplied by 96, should be the weight of the gun. Thus, a lightweight 12 bore should be 1-1/8 ounce load, times 96, = 6lbs 12 ounces or less.
Rule 2: The best shotgun loads are shooting a "square load" where the height and width of a shot charge are equal. This prevents damaging the static shot at the front of the load from being crushed and deformed by the moving shot at the back of the load during ignition. It also ensures the pellets have somewhere to go as they are squeezed first in the forcing cones and again in the chokes. A square load creates a wall of shot, rather than a string of shot.
Knowing those two rules, seeking overbuilt guns with long chambers undermines the whole plot. If you need a 3" 12 bore, you would be better suited by a lighter, better patterning, 2.5" 10 bore.
The big secret you should know if you want a nice 12 bore: 12 bore british shotguns are totally undervalued. A true "best gun" can be had far south of $10,000, whereas the identical gun as a 28 bore might be a $125,000 gun.