You should liken this investment to the investment required to make Tarmogoyf work, or Young Pyromancer, or Terminus, or Delver. Yes, you need to spend additional mana to make these things function at their peak, but that mana is being spent to do other things and the "flip" benefit is almost incidental.
There is a pretty big difference between Student of Warfare / Figure of Destiny that *require* and *lock you into* spending real mana to transform these creatures, and creatures like this that, if you build a deck that suits their purposes, can reach their peak with zero mana investment specifically used on them. If you're casting tons of dudes and attacking with lots of guys, you are gaining benefit from simply doing that, and any additional benefits you gain from flipping this guy are incidental.
Now, I'm not saying that this creature is as good as Delver / Goyf / Terminus et al. It remains to be seen whether a deck that plays a ton of attackers and is attacking each turn with 3+ creatures is really where you want to be in Legacy. But if that deck is possible and is viable, then there is really no extra cost involved in flipping this guy beyond his initial W mana cost, because your deck is just doing what it needs to be doing anyway - just like you don't view playing Brainstorm / Ponder / tons of other noncreature spells to grow Tarmogoyf and flip Delvers an extra cost, because you want to be playing those spells anyway.

