This creature not only makes your instant and sorcery spells cost one less to cast, but by sacrificing it you can copy the next instant or sorcery spell you cast that turn - and not just once, but for each time you've cast your Commander from the command zone in that game
In any case, this Tinker Bell, Giant Fairy card, at least in its Enchanted form, is worth around $250 on average
If your commander itself is a source of card advantage, then you can get away with a lower number here
She also doesnt get Medusad or Brawled and has a decent stat line
Heroic is an incredible ability to have on your creature, especially if your deck revolves around triggering mutate since heroic goes off as well
You need to have two spells this can tutor up for it to be worth it, and if you get there -- you're going to be really happy with this card