I think you’re underestimating the amount of effort just slapping things together would be if you want to come up with something so easy to use. Yes, something like this could be made cheaply using other parts put together, but I think it would be a significant project on its own. If you are working on a project and need an interface, starting another project to create the interface may not always be desirable. Before long you may find yourself in Tibet, shaving a Yak.
On the other hand, I do find myself hesitating to spend 2-4 times as much on an interface compared to what I’m spending on the actual computer. But that’s more because the Arduinos and Raspberry Pi are so darned cheap. Any sort of display for a RaspPi costs more than the RaspPi. Heck, even connecting a Pi to a non-HDMI monitor can cost more for an HDMI->DVI converter than the Pi costs (if you don’t shop around). I find this to be a difficult psychological barrier to cross. The 4D screens seem like a reasonable value for what they offer, if only the devices they connect to weren’t so much less expensive.
Spending $180 for a computer plus display doesn’t sound bad, but $35 for a computer and $145 for the display just seems wrong. But that’s not 4D or Sparkfun’s fault, it’s all in my head (and I suspect I’m not the only one).
