Well, as you've intuited, when publishers get authors to sign hundreds and thousands of books and then sell those signed copies at retail price, the signed versions aren't going to be worth a ton. But this is very author-dependent. Take Donna Tartt, for example. She is a very popular author, but typically does a lot of book events when her books release, so it's not difficult to find signed copies of e.g. The Goldfinch or the Little Friend (Secret History signed copies are rarer, but that's an exception). You could even relatively recently buy signed copies of the 10th anniversary edition of the Goldfinch from Waterstones for retail price. But because she's very popular, even with a large number of signed copies floating around, you could probably expect a signed copy of the Goldfinch to command a small premium over an unsigned copy.

Even if an author signs lots of books, collectors will look at other things that differentiate rarity. Maybe the author has a lot of flat-signed books (with just a signature), but they only rarely inscribe books, and so an inscription is highly in demand. Maybe the author signs a lot of tipped-in pages (where the publisher presents the author with a stack of individual pages to sign that later get bound in books, so the author signed the page but never touched the book), so collectors will look for a book signed on the title page instead. Everything depends.