Go to a music shop and plug a $4000 Les Paul into a $500 Crate half stack. Then go grab a $500 Les Paul copy and plug it into Friedman, Soldano, Diezel, Mesa, etc...half stack. You'll immediately hear the answer.
I'd even go so far as agreeing with the diminishing returns mentioned above as guitars go up in price. Take a $300 used Les Paul copy and drop about $200 in upgraded electronics in it. I'd bet in a blind test - live or on record - you'd find no one that would be able to consistently distinguish between it and a guitar that costs 10 times more.