The magic here is the Bluetooth temperature probes. Way cool. Utterly unnecessary, but way cool.
To make a BT temp probe, I'd use something arduino, not a Pi. Adafruit has a $30 board that has a Bluetooth radio and can run off a rechargeable Lipoly battery, All you'd need to do is wire a temp probe. I like 1-wire probes that do better than 1/8th degree F. (0.0625C) The Adafruit board even includes the necessary circuitry to charge the battery via micro USB. The battery and the temp sensor adds no more than $20. The real problem is making the case that can mount to the kettle. That might cost as much as the electronics.
$75 seems about the correct price point for a mass-produced, Bluetooth, kettle-mounted temperature probe. Add a premium for the Blichmann name and maybe $100 is not insane.
The data logging software can be done on any computer with Bluetooth, be it a desktop, notebook, Pi, or phone. Anyone who can program should be able to do it. In any event, it seems fun like a fun gizmo, but too pricey, even as a DIY.