@Robin129 Note that they say they use the 30-day "median" price. Median is different from "average". But no matter which one they actually mean, I can state with certainty that they're not really using either, at least not if by "30-day" they mean the rates set for the 30 days before the selected promotion dates OR the 30 days surrounding the selected promotion dates. I know this with certainty because of the odd behavior of a possible promotion I tested. Within my selected range, two adjacent days showed radically different "median" rates, one for $210 and the other for $135.
So, what I think they mean by "30-day median" is that, for any given day, they're using the rates you've had set for that date for the last 30 days. In my example, that could result in the discrepancy I noted. The day that showed a median of $210 (part of a higher-priced special event) had been set to my default rate of $90 and was blocked until about three weeks ago as it was too far in the future. After unblocking it, I set its rate to $240, then later changed that to $220. The date that showed a 30-day median of $135 had also been set to $90 while blocked, then set to around $160 when unblocked, then dropped to $140.
Long explanation, but I'm pretty sure that AirBnB is basing the "median" on the rate history of the selected date(s), not the existing rates of surrounding dates.