The Law of Equivalent Exchange is where things get messy, things get soft. Equivalent exchange is where the science of alchemy meshes with philosophy, with ethics, with metaphysics. It's not about concrete ideas like mass and chemical composition anymore, it's about meaning . Truth.
Alphonse has always been very good at equivalent exchange.
The thing about Truth is, it's not random. It's not unjust. It's not just, either, it simply is. What matters with Truth is causality. The punishment is always equal to the exact measure of the crime.
That is to say, Alphonse did not lose his whole body for no reason.
What is the value of a human soul? The answer is perfectly simple: a human soul. Nothing else could possibly be equivalent. This is as obvious to Alphonse now as it was when he was ten years old.
Edward's vice has always been hubris, and a straightforward way of thinking that can sometimes tend towards narrow mindedness. Ed did all the math, he understood all the chemistry, he drew the array with perfect exactness and made sure that they had precisely the right amount of each ingredient and not a milligram more or less. Ed did not believe in souls, or at least, not enough to account for them in his equations. Living things, people, weren't different from anything else, just very, very complicated and delicate. If you were smart enough and careful enough you could manage it just fine. No one had ever performed a successful human transmutation, but that was because no one had ever been as smart or talented as Edward and Alphonse Elric. Edward wasn't stupid, of course. He knew it could still backfire, he knew that he and Alphonse were risking their lives for this, and thought it was worth the risk. But he was confident, and so, so very hopeful.
Alphonse, too, was nearly certain that the human transmutation would work. He was also nearly certain that one of them would die for it. He was okay with that.
There is a difference between taking a risk and sacrificing, in taking a long hard look at your life, your brother's life, and saying, "Yeah. I'm willing to give this up in exchange for what I want."
Alphonse wasn't like Ed. To Ed, this was just science, dangerous and against the rules in the exciting way that so many things they did were. But Alphonse knew, soul-deep, that it was wrong .
Alphonse was just so tired, was the thing, so desperate and lost. Every day he felt his mother's face slipping away from him. He couldn't remember his father, and he was terrified of forgetting his mother too. He was tired of looking after Ed. He was tired of Ed looking after him. He was tired of only having themselves to rely on. Alphonse wanted his mom back. He wanted a home again.
Grandma Pinako was great, but Ed's stupid pride wouldn't let them stay with her and Winry, not permanently, not like family. And Alphonse… Alphonse couldn't trust it. Not quite. He and Ed weren't her real children, they weren't even her grandchildren. There was no reason to suspect that he and Ed would ever not be welcome at the Rockbell household, nothing except the harsh realities of life, that meant people did have to make cruel choices. You can't tell the future, can't know when hard times will come, and if they did, when they did, of course he and Ed would be the first to go. After all, Alphonse couldn't remember his father, but he knew his father had left them. It's what people do.
Alphonse missed his mother so much. In a world where the only permanent thing is impermanence, she had been stable, for a little while.
Alphonse loved his brother. He loved him more than anything else in this world. He would do anything for Ed, the two of them had each other and only each other and they put each other first in everything. But Alphonse loved his mother, too. Missed her like a drowning man misses air. And Alphonse was practical. He needed her more than he needed his brother, they both did.
Alphonse comforted himself, though, with the thought that it would probably be his own soul that would be sacrificed. It was he who had the intent to sacrifice it, there was little chance that Ed's would be involved. As though bartering with his own soul was alright.
Edward was young and optimistic, and Alphonse was young and foolish, and thought he understood the world enough to play within its rules. But, it did not turn out as Alphonse expected.
You can't get souls back. You just can't. Death is final. A dead soul is gone , and when it goes it cannot return. So Alphonse did not exchange his soul for his mother's.
The Truth is. Arguing with it is a pointless thing to do, but of course that has never once stopped Edward Elric.
Ed did not exchange his arm for Al's soul, because that would be ridiculous, and because you can't bring souls back . Ed traded his arm for the knowledge he needed to tether Al to the earth. Ed's sacrifice was a demonstration of intent for Truth, it showed how much Al mattered to him, that Alphonse's death would be as much a punishment for Ed as it was for Al.
Alphonse is alive because Ed deserved better. There is no other reason.
Ed is convinced he can fix them, that he can at least fix Al, he's determined to find the solution somewhere and refuses to give up. Alphonse is content to help him. If nothing else, it gives Ed a purpose, and more than anything Ed needs a purpose. And if, by some miracle, they do find their way back to bargaining with Truth, well.
Alphonse does not have much left. All he is is a soul in a suit of armor.
What is the value of a human soul? A heck of a lot more than an arm and a leg, that's for certain.