Spell cost vs Spell level

just a question, is the spell cost suppose to be the same for every level of the spell? Is there no reason to use lower rank healing spells? My memory is hazy but I do remember using lower rank healing spells to increase mana efficiency and minimize over healing.

Sometimes you use lower levels of a spell to cast it faster, but I don't think they should diminish in mana cost.

Good example is how frost mages use rank 1 frostbolt to fish for procs and snare a lot of players, faster, and from range.