Is this in the rules and I am simply not reading it correctly or documented errata? Not doubting you, just curious.
Which "this" do you mean?
The original question's answer is on page 8:
rules p8 wrote:
COST = 2 + number of symbols of the same resources produced by the brown and grey cards of the opposing city
The later question's answer is on page 15:
rules p15 wrote:
Starting on the following turn, you will purchase the indicated resource from the bank at the cost of 1 coin per unit.