On artist signature guitars
Manufacturers pay a certain amount to the artist for the right to put a name on the guitar. That money increases the cost of the guitar. Unlike normal production expenses, an endorsement contract (generally) has no effect on the quality of the guitar.
Les Paul guitars are a little different. Yes, it's his name on the guitar. However - depending on whose version of Gibson's history you believe - Les Paul probably designed the original model. At the very least, the R&D done by Les had a strong influence on the design. And we do know that he withdrew his endorsement when Gibson attempted to market the original SG guitar, because Les had concerns about the inferior neck joint.
And, of course, Les has been playing the electric guitar since the beginning - unlike many of the 'overnight success' players whose names manufacturers are now rushing to brand on the headstocks of their product.
So, I'd buy a Les Paul... if I happened to like single-cutaway guitars, which I don't. I support Les Paul's endorsement out of respect for a man who helped shape this industry. When it comes to supporting someone who's "just" an artist (not to imply that's less valuable), I'll buy their music and leave the overpriced copies of their instruments to people who like that sort of memorabilia.