> "It’s not entirely clear to me why the Beagleboard is so expensive ... "
We are told the Pi took 6 years to develop. I'm guessing that during the time the developers had proper jobs and regarded the Pi as a sort of altruistic hobby. It definitely wasn't going to be a source of income during those years.
Consequently all the time and resources used for the development process are a sunk cost and don't have to be recouped from the unit-price of the eventual product. That's what makes the Pi different from commercial offerings. In these cases the years (or more likely: months, for time is money) of developer time has to be paid for - in salaries, equipment and facilities.
We also know that given a large enough production run (say for a mobile phone) these costs don't add a great deal to each board when you're producing a million of them. Even less if all you have to do is add new features/power to an existing design. However for a low-sales, niche market that only produces one-hundreth the number of units, those same costs will contribute 100 times as much to the price of each board made.
Maybe the longest lasting legacy of the Pi won't be introducing children to little motherboards, but will be the creation of a low-cost, open sourced basis for future embedded hardware.