Phew - just looked at the data sheet. Full brilliance for a whole sheet consumes 3.84Amps.
Multily that by 5 or 6 sheets means a hefty PSU or multiple feed-in-points.
Well that's only 19W (3.8A * 5V) per sheet. If they have the same characteristics as other WS2812 LEDs I have, I would probably run them at 10-20% of that figure.
Looking back - I think the price I saw was a new-shopper-introduction (1-per-order).
Just looked again - price has indeed increased to £2.17
They must have "sensed" there might be an upcoming demand and increased the price.