Of my collection of firearms--some of which predate the requirement for serial numbers--I have exactly three which (to the best of my knowledge) have been modified:
1903 Springfield: modified before purchase. When the NRA was selling surplus Springfields, most were modified by innumerable gunsmiths across the country before sale. Mine had the barrel and stock shortened, and possibly a change of sights. I removed the rear sight and installed a no-drill scope mount. The gun was already bubba-fied, so I figured I may as well toss the history and make it useful.
Colt Combat Commander: Jim Clark (the real Jim Clark, not somebody employed by him) did a trigger job, sights, and I don't know what else some twenty-five years ago. I acquired it in this condition, and wouldn't change it for all the tea in China.
Ruger Security Six: my dad changed a couple of springs.
By your definition, my Dan Wesson 1911 is also modified. I take exception to your definition, mostly because $600 doesn't go very far these days. I paid $679 for it (if memory serves), and consider it a steal at that price.