Grand total it has 4 extra buttons, 2 capacitive strips, 2 touchpads, gyro, and the fancy magnetic joysticks. $100 did not surprise me at all.
Of course, if you don't care about any of those features, it isn't the best deal, in the same way that a luxury sports car makes for a shit commuter vehicle. Not that the steam controller is "luxury" but like, if you don't want the extra features then the higher price probably isn't worth it. That doesn't make it overpriced, it means you aren't the target audience.
Full disclosure. I'm that target audience. I'm already mentally mapping extra buttons to the left touchpad and thinking about setting the back buttons to my armored core's shoulder weapons.