I went to Best Buy looking for a DVI <-> HDMI cable once. They wanted 40 dollars. I got it on Amazon for 7.95. And they wonder why their stock recently took a big hit...
They're also great for adapters. I bet 10% of the people reading this have paid Apple $30 for a Mini DisplayPort to {VGA|DVI|HDMI} adapter. I got all 3 for $30-something.
Though cables can vary a bit in price depending on quality, so that might have come into it a bit too.
(for those who are unaware: basically the entire purpose of digital communications is to confer immunity to signal noise and distortion)