Th $50 ones were made for the iPod touch's. The cheaper ones were made for the iPod Video.
You used to have to use the more expensive with the Ipod Touch or you would not get video only audio. People that bought the cheaper ones were pissed, to say the least. So when they upgraded the firmware to 1.1.3, they changed it so you could use the cheap ones, too.
just to be sure, though, hit up the macrumors or insanelymac forums.
and, what kind of iPod do you have?