So, over on the group messaging channel, we have been discussing, a lot, about the BoM - the Bill of Materials before we even bring out the notepads to make the schematic sketches. This is important because it helps us to make sure our design will meet one of the main criteria we set forth in the beginning - cost. Here's what I've distilled from all the chatter :

Microcontroller : After quite a bit of discussion, we finally zeroed in on the Atmel ATSAMR21 - a microcontroller + radio rolled in one. A 32 bit ARM Cortex M0 with a 2.4GHz transreceiver.

Battery : A 3.7V, approx. 2500mAh battery will power the badge. Estimates tell us the display will gulp about 100mA, and the rest of the circuit an equal amount. This is erring on the higher side a lot, so the battery can be expected to last at least 10 hours on a single charge. Enough to last a whole day before needing a re-charge.

Display : We looked at e-paper and Sharp memory displays, but will most likely settle for a 2.4 inch, color TFT LCD display with a 320x240 pixel resolution.

These three are the big ticket items on the BoM. Choosing the cheaper LCD makes sure we limit the BoM cost to around $35. With the e-paper and Sharp Displays, we touch almost $50 on the BoM.

Besides the above three, the Badge will likely have the following parts :

Several WS2812B RGB smart LEDs

A speaker, low profile, capable of playing some low fidelity audio

Capacitive touch buttons formed using the Hackaday Logo

A pair of IR transmit and Receive LEDs. These could have several applications - interacting with other badges, allowing programming of badges using blinking clock and data squares on a host computer etc.

A Li-Po charging regulator

A buck regulator to produce the 3.3V required for most of the board, and another 5V boost regulator to run the RGB LED's and the OTG USB connection.

Connectors - Mini or Micro USB, JST for the Li-Po, headers for programming etc.

Footprints and headers for mounting stuff like LDR / Photo Diode (ambient light sensing), Acelerometer (pedo meter), headers for HC-05, ESP8266, GPS, etc.

Add in passive components, the PCB, Assembly, Testing and related stuff, and we are pretty close to the $30 sweet spot.

You'll find the "work in progress" BoM at this spreadsheet link.

Any thoughts, chime in with comments below.