Against the board’s own history. The usual price is the time-weighted median of the prices we have recorded for that board over the last 90 days: the price it has actually held for longest. A board is listed when it is 10% or more below that.
Never against the RRP. A “was” price based on the manufacturer’s list price says little about what a board really sold for, so we ignore it.
New boards. Until we hold 14 days of history for a board we fall back to the was-price Amazon itself reports, and say so in the table.
Prices move. We check every six hours. The price shown on Amazon.co.uk when you buy is the one that applies.