C.S. Lewis never explains what the price was in the books or otherwise. The "where" is equally vague, but we can reasonably glean that it was not learned from a (still living) person, but rather a secret learned by finding the place that secret was hidden, and that learning it cost her something of great value to herself (note that she doesn't even say that learning it is what costs a great price: just that her journey to learn it did).
Here's pretty much all of the relevant quotations regarding the Deplorable word by Jadis in The Magician's Nephew (emphasis mine):
"She even knew that I had the secret of the Deplorable Word. Did she think—she was always weakling—that I would not use it?"
"That was the secret of secrets," said Queen Jadis. "It had long been
known to the great kings of our race that there was a word which, if
spoken with the proper ceremonies, would destroy all living things
except the one who spoke it. But the ancient kings were weak and
soft-hearted and bound themselves and all who should come after them
with great oaths never even to seek after the knowledge of that word.
But I learned it in a secret place and paid a terrible price to learn
"Then I spoke the Deplorable Word. A moment later I was the only living thing beneath the sun."
Then later, from Aslan (pretty much the only other place the word is discussed).
"It is not certain that some wicked one of your race will not find out a secret as evil as the Deplorable Word and use it to destroy all living things."
From these we can deduce that the Deplorable word of her world was something locked away for many generations, and seeking to know/knowing it was forbidden, and thus there was likely no one left alive who could have taught her. While it only explicitly mentions the ancient rulers, it seems probable that they were the only ones with the authority to learn it without consequences, so they only had their oaths to go by, and as such were the only ones worth mentioning.