35 packs worth of gold, but with a guarantee of X/Y/Z legendaries/epics/rares/. Looking at One Night in Karazhan it was 2800 gold (4x700) but gave 2930 dust not counting the free introductory wing. Which puts it at 1.0464 dust per gold compared to the average 1.0271 dust per gold from an average pack. That's just nominal disenchant values not the amount you save by getting the card you actually want instead of crafting it.

Values from https://hearthstone.gamepedia.com/Card_pack_statistics so they might be off, but I think your undervaluing expansion/ overvaluing packs.