The main gold cost comes from trying to max the level of your units for awakening. A unit must be max level as a prerequisite (even if it wasn't a prerequisite, it would like CC'ing anyways: you wouldn't want to awakening anything other than a maxed unit). For example, depending on what fodder you use it can cost up to ~2mil gold to get a Black unit from level 70 to 80 (not counting respective fodder's mission rewards, just raw amount).