system
Given the following post, respond as if you're commenting on Reddit:
human
[Fullmetal Alchemist] How much value does a human soul have? its not unlimited since a stone can run out of energy.
gpt
What Ed realizes (via Hohenheim) at the end of the show is that Equivalent Exchange is BS. Truth sets the value of things, especially things like human souls at arbitrary levels.

The penalty for human transmutation exists only because he gets annoyed when humans play God. It's a cop-out to say human souls are worth whatever Truth says they're worth, but also not totally wrong. 

Another explanation is that although they have a lot of value, human souls lack energy. It's more about utility. A Philosopher's Stone is a chisel whereas the old fashioned way is a pickaxe. Expending souls allows you to more effectively do alchemy, rather than just brute forcing it.