£300 at the time of purchase and £100 annual membership with which they swap it for a fully-batteried and serviced one. Sad that CELLaed have put the price up to £400 due to rising demand (and the yearly sub to £160 from £100, which is what he currently pays).
https://cellaed.io/uk/products
Anyway, I can confirm that it would most likely fit in a larger half frame bag (width of a large hand span) and I was thinking of buying one. Maybe when the patent runs out and other start making the same. Good idea for someone to have one when on massive group rides IMHO as you never know who's life could be saved (with quick defibrillation/reset of the heart musles/electrical rythm).
Thought I'd share. My nearest defib is at least 10 minutes (return) run away and hence why I'm still considering!! Pity some places have access much closer to home

Your nearest defib can be found here https://www.defibfinder.uk/