All Magic has a Cost

Background

In this post I thought it would be interesting to discuss an SSM where GRRM discusses the role of magic in the ASOIAF universe. I find his use of magic amazing. He does such a great job of using it, but also keeping it on the fringes of the plot action (especially in the south). He also doesn't have hard set rules for magic (like other authors do) and seems to play fast and loose with whatever best fits the story.

If interested: All Magic has a Cost: A List of "Every" Example & The Fallacies of Prophecy & Sorcery

From GRRM at Russia's World of Fantasy in 2017. This is translated from Russian, so please let me know if any of my wording is off:

If you look at my books, I’m not particularly revealing the secrets of all these processes. I basically do not like the system of witchcraft. At conventions I am often asked how to create systems of witchcraft, and I answer that they do not need to be created. Here you are not at Hogwarts to have a mentor teach you to resurrect people - and students would get fives, triples and twos depending on the results of the resurrection. This is a secret art, supernatural, dangerous, we are talking about the powers and deities with whom it is better not to get involved. 
One of my heroes says that witchcraft is a sword without a hilt, you cannot grab it and do not cut yourself. Here is the magic I try to describe. Of course, partly the magic that the characters use is the focus. Melisandre has magical powers, but her potions and powders that catch fire at the right moment are the same witchcraft as the illusionist in the circus. Apply all this systematically will not work. So, the intrigue remains -SSM, Mir Fantastiki 2017

I think this quote exemplifies what GRRM has done in the series. There is magic (with no defined set of rules outside of there always being a cost), but there also exists allusions (potions, powders etc) that are used to trick and seem more powerful than they actually are.

If interested: All Magic Has a Cost: A Focus on the Weirwoods/"Northern Magic"

All Magic has a Cost

This fast/loose rule that GRRM uses is repeated over and over in different ways:

"Old Nan says the children knew the songs of the trees, that they could fly like birds and swim like fish and talk to the animals," Bran said. "She says that they made music so beautiful that it made you cry like a little baby just to hear it."
"And all this they did with magic," Maester Luwin said, distracted. "I wish they were here now. A spell would heal my arm less painfully, and they could talk to Shaggy dog and tell him not to bite." He gave the big black wolf an angry glance out of the corner of his eye. "Take a lesson, Bran. The man who trusts in spells is dueling with a glass sword. As the children did. Here, let me show you something." He stood abruptly, crossed the room, and returned with a green jar in his good hand. "Have a look at these," he said as he pulled the stopper and shook out a handful of shiny black arrowheads. -AGOT, Bran VII

and:

Sometimes the short road is not the safest, Jon Snow. The Horned Lord once said that sorcery is a sword without a hilt. There is no safe way to grasp it." -ASOS, Jon X

and:

Dalla told me something once. Val's sister, Mance Rayder's wife. She said that sorcery was a sword without a hilt. There is no safe way to grasp it."
"A wise woman." Melisandre rose, her red robes stirring in the wind. "A sword without a hilt is still a sword, though, and a sword is a fine thing to have when foes are all about. -ADWD, Jon VI

and:

"Born amidst salt and smoke, beneath a bleeding star. I know the prophecy." Marwyn turned his head and spat a gob of red phlegm onto the floor. "Not that I would trust it. Gorghan of Old Ghis once wrote that a prophecy is like a treacherous woman. She takes your member in her mouth, and you moan with the pleasure of it and think, how sweet, how fine, how good this is . . . and then her teeth snap shut and your moans turn to screams. That is the nature of prophecy, said Gorghan. Prophecy will bite your prick off every time." -AFFC, Samwell V

and:

Did Benerro see this in his fires? Tyrion wondered, when he realized the huge red priest was gone. Did Moqorro?
"Prophecy is like a half-trained mule," he complained to Jorah Mormont. "It looks as though it might be useful, but the moment you trust in it, it kicks you in the head. That bloody widow knew the ship would never reach her destination, she warned us of that, said Benerro saw it in his fires, only I took that to mean … well, what does it matter?" His mouth twisted. "What it really meant was that some bloody big storm would turn our mast to kindling so we could drift aimlessly across the Gulf of Grief until our food ran out and we started eating one another. -ADWD, Tyrion X

Only Death Can Pay for Life

As an extension of "all magic having a cost", I think we can take that a step further with another step GRRM seemingly requires:

Only death may pay for life. -AGOT, Daenerys X

and:

"The Red God has his due, sweet girl, and only death may pay for life. This girl took three that were his. This girl must give three in their places. Speak the names, and a man will do the rest." -ACOK, Arya VII

and:

"Only death can pay for life, my lord. A great gift requires a great sacrifice." -ASOS, Davos V

If interested: Only Death Can Pay For Life

and keeping with the cost mantra, it should be noted that death changes you:

Death is hard." lol.gif The character gets more and more removed from his or her former life. The main thing remaining, what brings Beric back, is the sense of purpose, the mission he has yet to accomplish. - SSM, US Signing Tour, 18 November 2005

the normal pushback I get is what about Beric's first death, I would say (remember fast and loose magic rules) this counts:

We had lions on every side, and I thought I was doomed with the rest, but Alyn shouted commands and restored order to our ranks, and those still ahorse rallied around Thoros and cut our way free. Six score we’d been that morning. By dark no more than two score were left, and Lord Beric was gravely wounded. Thoros drew a foot of lance from his chest that night, and poured boiling wine into the hole it left. -ASOS, Arya III

this can also be applied across all types of resurrection/egg hatching rituals as well:

There were dragons here two hundred years ago, Sam found himself thinking, as he watched the cage making a slow descent. They would just have flown to the top of the Wall. Queen Alysanne had visited Castle Black on her dragon, and Jaehaerys, her king, had come after her on his own. Could Silverwing have left an egg behind? Or had Stannis found one egg on Dragonstone? Even if he has an egg, how can he hope to quicken it? -AFFC, Samwell I

as we should expect Stannis to attempt to do with Shireen.

TLDR: In 2017, GRRM mentioned how he did not have a system of magic for ASOIAF. Instead it seems he plays fast/loose with the magic (while keeping it on the fringes on parts of the story) for the best narrative possible. This allows him to still keep with "sorcery/prophecy being dangerous" (all magic having a cost) without committing to strict rules.