Of course it would need additional board for keyboard attached to PI. The size/bulkiness wouldn't be so bad, here are some photos:
The position how it could be done - hdmi and audio would need to be rerouted by cable extensions to top (hdmi) or upper left (audio). Red rectangle is keyboard board attached to gpio on the left, blue is space for battery.
Okay, looking at the power consumption of the Pi2, it needs about 300mA when using a single core at 5V.
That would be 1,5W. The Display would need about 0,7 W as well.
So you'd have a power usage of 2,2W when doing normal work. This is without Wifi or anything else.
A battery with the size you are planning would probably have 2200mAh with 3,7V, so we'd have about 8,2W.
So with simple usage, you'd have a battery time of about 3,5 - 4 hours, less when you use more cores or have additional Wifi, etc.
The thickness at usb port is 2cm (red), thickness of the rest of the Piu board where the keyboard board would lie is 1cm (green):
Don't forget you also need some hinge for the LCD, etc. somewhere.
Of course one custom board designed directly for the device will always be better but this is about compromise and reusing existing solution to save the price and piggyback on Pi success and big community.
So instead of whole device this would still mean custom case, keymat, lcd, keyboard board with lipo charger and controller chip (some arm cortex like in Teensy or Maple) sending key events and doing power on/off. So it looks like most of custom costs are still there but the board is much simpler and existing PI OS/software would be easy to reuse so one could save on the development. Also if it would be sold as a kit/docking station maybe it would not need some certifications (wireless) or extra fees (sd, hdmi, codecs)? If this could be made to be sold for 200EUR it would make sense to me. But if you say it could shave off only 150 EUR (from 500-600?) it would still be very expensive indeed.
All the custom stuff makes things expensive. You also need some space for all the chips, and an issue with reusing existing solutions often is the power usage.
You can't optmize the traces for the power circuit, etc., so you probably have a higher power consumption that you could have.
Of course it would not be as good/fast as Pyra but the quad core with the GPU and other cores the PI 2 has is still plenty for emulation and much better than Pandora. And there is lot of optimizations on the software side and agreements with various third parties done by Raspberry Foundation which would bring additional value for such device.
True. But I really don't think that could be done in an ergonomically useful way for a good price.
EDIT: As for cost, here is something similar but a bit larger
https://www.indiegogo.com/projects/pi-top-a-raspberry-pi-laptop-you-build-yourself#/storyThey estimated it to 299USD including the pi board, and larger batteries/lcd but not custom keyboard, only the case.
Well, that it totally different.
They have a VERY simple case design and just leave a hole for the keyboard.
Instead, they include a simple standard keyboard that can be gotten for less than 3 USD (probably) in China.
The only expensive part here should be the LCD and the case, there's nothing else here.
With that size, they don't need a custom LCD cable as well.
LCDs that size are also cheaper than small LCDs with high resolution.
That stuff all adds up. Keymat, for example is 10,000 EUR for the mould and 8 EUR for each mat.
LCD Cable is actually cheap with the Pyra - only 2 EUR.
Case moulds are about 50,000 EUR.
And don't forget the development and design cost as well (that was probably 100,000 EUR).
With a simple design as the PiTop, it's easy to keep the costs low.
Well, in my opinion, 299 USD is already pretty high for a case and an LCD with a standard keyboard and a battery, at least if you expect at least 1000 units.