Most of the guns that were made were cheap guns.
Where is your evidence? The ledger books of gunsmiths show $11 & $12 guns, but no mention of "sold two $5 rifles off the factory-seconds discount rack today".
Now lets compare pianos. People that are apt to buy a new piano want a darned good piano. A company that throws cheap pianos out the workshop door doesn't stay in business long. The companies that hand-craft fine pianos are back-ordered for months. You want to spend less? Buy a used piano.
"Cheap" modern guns are mass-produced on replicating CNC machines with very little hand crafting. Rifling was done by hand, one groove at a time. You can't compare a hand-made flintlock to a J.C. Higgins.
In the 1700's a gunsmith lived or died by his reputation, and he had little interest in swapping volume for quality if he was making ends meet. The materials were worth much more than his and the apprentice/shop boy's labor. Just the opposite of today's gun shop.
In the American Revolution there came a desperate need for weapons, and a lot of muskets were thrown together. Thereafter, a "cheap" gun would likely have been one of those or a used up issue Bess.
I believe many of the "barn guns" were assembled from old parts, and the dates fall into the 1830's or thereabouts - when the country was in a bad depression and materials were hard come by. That would put the date of the assembly long after the parts were dated, and probably not by the original smith.
Again, just conjecture for discussion.