"Spent to Cast," aka Ogre Savant Ability, aka Sunburst, aka cards in the Shadowmoor/Eventide cycles including River's Grasp and Moonhold. These cards care about the mana that was spent to cast them. Sunburst doesn't care what mana was spent so long as it's different and it provides one of two set effects related to that. That makes it different enough that it discussed in depth elsewhere.
Spent to Cast encourages splashing other colors. It usually requires mana of one color to cast, but gives a bonus if another color is spent. It's popular on instants and sorceries and when it's used on creatures, it often confers a one-time effect , even if that effect is just to keep the creature, in the case of the Simic, Azorious, and Rakdos implementations (where Spent to Cast acts like strange Evoke instead of a strange Kicker). This avoids memory issues in much the same way that Sunburst does with counters.
Cons
The spell taped to a creature issue sometimes results in cross-color abilities: Court Hussar has a very blue ETB effect, but has a white ability, one you only get to use if you pay white mana for it. While it's an interesting approach, it ends up being a blue card with vigilance, just like Azorius Herald is an unblockable white card and Crypt Champion is a black card with Double Strike. That particular approach to the color pie might not work in a core set.
Crypt Champion itself raises an interesting point; while Crypt Champions can't reanimate one another, these strange evoke creatures are very resistant to being cheated into play. This might discourage new players and even cause uncertainty when the appropriately colored mana is spent on an ability that puts them into play (Quicksilver Amulet or Birthing Pod). This is either an opportunity for education or a dangerous point of confusion.