I don't think I've paid a fortune (or anything much over list price) for a book or score, but I definitely have a few things that are worth much more now.
One of my better book "holdings" is a huge 3-volume Chinese-American dictionary published early in the last century. We have some Chinese artifacts in the family and it attracted my interest at a library book sale where it was priced at $100. I went home, did a bit of research on it, and decided if it was still there the next day, which was to be their half-price day, I'd buy it. It was, and I did. It flew right under the radar. For my $50 I got what seems to be an exceedingly rare volume, a lessor edition of which was priced at $2K-$3K online, and this was a good 15 years ago. I should check it again.