Well, except ancient Imperial Jewel and Prismatic Orb, whose mana cost can be considered as “already doubled” for its relative weak effect.
Reason: more mana colors means more chance of filling, regardless ordinary matches, conversion matches, wildcard matches (takshaka / centuragon), elemental / umbral star matches (wand of stars), or mass destroy / explode gems (thrall and his clones, or explode-summon weapons). Especially with later options, filling 30 to 34 mana of 6-colors ultimate dragonite dragons are about doubly easier than 28 mana of their regular cousins.
Comparing similar spells from common troop (single color, 6 to 8 cost) with rare / ur troop (two color, 9 to 12 cost), we can see a roughly 50% cost increase. Similar increase also can be found from legendary (two color, 15 to 17) to mythic (three color, 22 to 26), roughly 50%, too. Following that trend, “4 color troop” cost 50% more than regular mythic, “5 color troop” 50% more than “4 color”, that’s already more than twice of regular mythic. So 6-colors ultimate dragons have their cost doubled isn’t absurd at all.