I didn't read 5 pages so maybe this has been covered....
I had the same mentality, I had sub $1k O/U's because they were "just as good" and pretty enough for me. Then when I got into the shotgun sports, and regularly ran 300-500 shells a week through them, the difference started to show. I had to replace firing pins, springs, from just wearing down from use, then the hinges and locking lugs started getting loose. So much that, when the action is closed, if you grab the shotgun midway and shake it, there's a distinct wobble. Then I got into trap. I didn't have thousands to spend on a dedicated trap gun, hell, even nice used ones can run $4k pretrty easy. So I got a BT-99, great dedicated trap gun. Its from the 70's, fixed full choke, and allready had the adjustable comb, butt, and helicoil recoil reduction set installed, and I paid $1,500 for it. Lord knows how many rounds that thing has seen and its tighter than any of my newer doubles and I smash clays like a champ with that thing.
So, in short, I'd say build quality/materials. Many of your typical Turkish imports are fine guns and will give you years of use for the typical hunter, clays shooter. But put them in the hands of someone serious about clays and runs thousands of rounds a year through it and its a different story.
Price wise....once you get into the B guns, your fine, so say $2-3K for a Citori, then ad on the customization's as you go along to see what you like and don't like. You can buy a Citori already set up, but at a much greater cost as opposed to having a pro customize it and usually do a better job that the factory. Once you get to a certain point I think it just comes down to window dressing like nicer bluing, exhibition grade wood, elaborate engraving and such more so than importance of function.
I'm not a pro or even that smart, that's just my .02