( some people pick resistances accordingly to legendarys that only give a certain resist like Seven Sins )
So be paitient, none is cheaper then another, because according to blizzard each res attriubte have equal high chance to get rolled :) It's just a matter of rng, prizes vary because of this rng.
Our contenders for pieces, demonhunters. Do not direcly search for dual res in particular, so they go for your item nomatter what the extra res is, if the other stats are nicely rolled.(damn thoose demon hunters -.-')