In general, I find this sort of "reduction in value" from a working product to reusable parts sorta depressing. But I guess that's looking at it backward; better a few cheap parts than a pile of nameless eWaste.
I think you are looking at it wrong; its like looking at wrecked or junked/rusting automobiles, and thinking they're nothing but a bunch of scrap metal.
Meanwhile, tons of pick-ur-part places make a lot of money off those same vehicles (and whatever is left gets sent to a recycler scrap yard).
Generally - marketed properly - you can take "junk", take it apart, sort the parts and such, get enough of them and sell them as used components for hobbyists, if you wanted to. You might get something for free, or buy something at goodwill or a yard sale (for a few dollars) - tear it down, sort the parts, package them up, and sell them to others (who either don't or can't do what you did). You could, in theory, turn a $5.00 "junk" device into $10.00 of parts.
Here's a "for instance" - priced the Senscomp ultrasonic kits?:http://www.senscomp.com/products.htm
Kinda expensive, huh? But they're pretty nice kits - in some ways better than the PING-type sensors, but not in price.
But there's a cheaper alternative, if your willing to do the work to remove it - from a Polaroid camera (Sun 660, Spectra, plus some others). You can find these all the time at yard sales and goodwills, generally for less than $10.00 (most of the ones I've found have been for $5.00 or less).
I bet - if you removed such a sensor from the camera - and set it up to be controlled by the Arduino (few parts are needed for this; it was detailed in Servo magazine not too long back by the member of SRS who originally showed how to use such devices with a PC parallel port) - with testing, packaging, marketing, and maybe some code - you could probably sell them for somewhat more than the PING-style sensor, but waaay less than the Senscomp kit, and easily make a healthy profit, while providing a useful device for hobbyists (or students, or educators) at a more affordable price.