Lempkee: There are many variables affecting final board manufacturing cost.. Like number of pcb layers, board size and tolerances on manufacturing... (Via and drill sizes, trace witdhs etc.. ).. Board size itself ain't the major price factor (and smaller is cheaper if everythng else holds the same).. But if one has to use multiple additional layers for routing on undersized board (like those Blizzards for A1200) then size becomes costly.
But.. Quite frankly board manufacturing prices have never been real issue on those AmigaOnes.. Real cost comes from extra middlemen between board makers and users.