The thing that makes Emerald Shield and Reflective Shield unique is its spell-stopping power. (I don't consider the immortality unique because Enchant Artifact has the same effect.) However, this only applies to spell DAMAGE, and there is rarely good reason to attack anyone with spells, as they are more effective at stopping creatures to give you more time to get out creatures that will deal more damage, or will have a stronger effect on the field. Even the highest damaging spell can at 75(!) fire quanta only deal a measly 24 damage. Against players, you'd need 4 + something to finish (eg Fahrenheit in this case), and when you're doing arenas or lvl4s it's not going to cut it.
Then there are psions and Shard of Wisdom, which is the only spell damage strong enough to be considered a problem. However, these are still fairly rare, and putting in a card purely to counter an 'occasional' problem might slow your deck down and lower your winrate against other decks.
As such, the spell-damage-reflecting power is what is UP and deserves a rework; for example, for it to make you untargetable to enemy spellcasts. (Psions and SoW'd creatures could simply not attack as a slight nerf/balance.) Or for an (x) quanta cost, to make your creatures / a creature untargetable for 1 turn. Either way, this would obviously go with a lower damage block in the case of Emerald Shield and a higher quanta cost for Reflective Shield.
tl;dr right now, the only special jig about Emerald and Jade shield is the innate immortality, and the spellblock is often useless. Reducing cost would only change it into a Shield or Towershield with immunity to PC, which shouldn't be what these cards are about. The unique thing about these cards, the spellblock, does deserve a mechanics rework.