sure, overall the cost of the computer board may be a small part of the overall construction cost, but if it's a hobby, it's still cash out of your pocket (and if you are looking at adding a computer to something you are already doing, be it model boats, rockets, or whatever, your view of the costs related to those things tends to ignore those, as you would be spending that sort of money on your hobby anyway, even if you didn't add the computer)
If you're looking at a commercial product, the $100+ difference in per-unit costs is a LOT.
the fact that 'developer' boards are considered 'cheap' at $2K each is highway robbery by the chip manufacturers., yes they don't sell a lot of them, but they had to develop the 'developer' or 'prototyping' board anyway for their internal people to test the chip on, once they have done that development effort, the incremental cost of producing more boards to sell to the public is minor (and tending towards insignificant with more flexible assembly lines)
so being able to get a functioning computer for $35 instead of a developer board for $2K (or in the case of ARM, a stripped down system board for $150+ that needs another $100+ board to talk to a network, and a $100+ board to talk to a display... etc) is huge
The arduino is a joke as a computer nowdays, but it's so cheap (along with easy to do simple things with), that it gets used a LOT and considered by people who wouldn't even think of using a computer in something at the traditional prices.
At $35 the RPi board is in the same price range as the Arduino (and it includes USB, network, and display!!). It's not yet as easy to program to do things, but I'm expecting that there will shortly be simple software packages to configure Linux on the RPi to drive Arduino add-on boards, and once you can drive those boards from within Perl or Python, and not have to do the compile/load/test cycle of the Arduino, I would expect that the RPi will make a HUGE dent in the Arduino community.