That's the whole point of Path of Exile's economy. It's based on barter, there is no "fixed value", it's all about exchanging, and the value something has for you or for the buyer.

From the [official page][1]:

> Path of Exile is an online Action RPG set in the dark fantasy world of
> Wraeclast. We're a small independent team of hardcore gamers based in
> New Zealand and have created Path of Exile as the game that we'd want
> to play ourselves. **It is designed around a strong barter-based online
> item economy**, deep character customisation, competitive PvP and ladder
> races.
> 



To read further, here is an extended [Dev diary about economy][2].


  [1]: http://www.pathofexile.com/
  [2]: http://www.pathofexile.com/forum/view-thread/55102