My experience is based on SxSs, but O/Us follow the same rules. So I'll just call them "doubleguns"
There are plenty of good doubleguns that are inexpensive. They work. But a doublegun needs to have the barrels "regulated", that is, adjusted to shoot to the same place. That is a difficult labor-intensive job to do well. An inexpensive gun may not be well-regulated. Indeed, on SGW there are several ongoing threads right now that bemoan this fact. An expensive doublegun will generally not have regulation problems; an inexpensive one could.
But what really is an elusive issue is the "feel" of the gun.
If you shoot a pump or semi-auto, they mostly feel the same except for possible differences in recoil. (Please note - I assume that any gun you shoot fits you so it is immaterial if you don't like an "X" gun "because it doesn't fit" If it's a good gun it's worth restocking)
Now doubleguns have distinctive "feels" to them. The way they mount and swing; their motion dynamics - all of these very subjective qualities that exist in a gun. It is not possible to describe this - but if you ever get the chance to shoot a Baikal and then a Boss O/U you will immediately know what this is all about. My 35-yr-old F. Anitua Spanish SxS kills as many clays as my AyA but the experience is not nearly as satisfying.
I shoot SxSs only. I have some superb guns and I also have some very inexpensive older Spanish SxSs that literally cost me less than $500 each. They are fun guns - I don't really care what happens to them. They fire each time I pull the trigger and al of them are well-regulated. I have re-stocked many of them so they all fit me perfectly.
However, they lack that certain "zing" that makes just mounting and swinging a "best" so wonderful.
A fly fisherman once smiled at me and told me he began to really like fly-fishing when he realized that fishing was not about catching fish.
Same for doubleguns. It takes experience and some good coaching to learn how to properly mount and swing a gun and once that's down then the difference between an ordinary doublegun and a good one is the difference between croaking and singing.
If the guns all go "bang" then the only thing more expense gets you is the "bling" that you feel when you use the gun.
And it is that "bling" that makes a really fine doublegun so wonderful.
A Hyundai will get you home, as will a Jag. You pay for the experience.
(flamers- please note this is a statement about guns, not people or their ability to afford guns! It does not involve engraving, wood quality or wood/metal fit. No social statement of any kind is intended.)