Original card.

Continuing the theme of toughness matters. I know the wording is a little weird. I'm pretty sure it works; targets are chosen before players pay the cost to cast a spell. So the player first chooses which creature they are enchanting, then looks at that creature's toughness and reduces the cost.

The fudgy-ness comes into play when I opted to avoid using the word 'target', since I don't think many players realize they are targeting when they are enchanting. The grammar isn't perfecting, but hopefully it gets the idea across without having to lift the mechanics curtain.

As for the cost, I wanted , since that's what Abbey Matron does on activation, and using the highest toughness common as your stopping point seemed like a fair guide. But I'm not sure if I made this potent enough, or if it's already a bit much. Both lifelink and protection from instants were considered... but I thought I might be going a little too far for an aura that could regularly cost with little effort (I should point out that many of these toughness matters cards combo quite well off a single Giant Growth.)

On the art: I hate using WoW. But sometimes, it's spot on.

I'm pretty sure "enchanting" can't be right. "enchanted" possibly. But I think my response to "players don't realise that Aura spells target" would be the opposite: make it deliberately use the word "target" so that players learn. See the reminder text on "enchant" the most recent time it had reminder text such as on Treetop Bracers.

Also it was more or less assumed knowledge in Theros block

Both fair points. I'll switch over.

Switched to "Blessed be the Righteous costs X less to cast, where X is equal to the targeted creature’s toughness."

...but that's not quite right. That sounds like it could be targeting any creature, not necessarily the creature it's enchanting. I guess that was my real problem all along... not that I was afraid that people wouldn't know that auras target, but that they wouldn't make the proper mental jump to associate that rule with this card's text. Maybe I should put aura reminder text just on this one card? Would that do anything?