I am providing context from the books as you said those were acceptable. The transaction is a bit different there (as is most things). The Good Masters do not sell Unsullied in groups of less than a 100 but prefer to sell in groups of 1,000. So from the beginning it difficult to determine the value of a single Unsullied.
Daenerys' interaction with the Good Masters is not a good example or fair trade. Daenerys wants all the Unsullied. The bulk of ther interaction comes from A Storm of Swords, Chapter 26.
"All," growled Kraznys mo Nakloz, who smelled of peaches today. The slave girl repeated the word in the Common Tongue of Westeros. "Of thousands, there are eight. Is this what she means by all? There are also six centuries, who shall be part of a ninth thousand when complete. Would she have them too?"
"I would," said Dany when the question was put to her. "The eight thousands, the six centuries . . . and the ones still in training as well. The ones who have not earned the spikes."
The Good Masters are greedy and read the situation that Dany is desperate:
"We will tell him that he must wait," said the fat man. "Gold in my purse is better than gold in my future."
The slavers disagree on if they should sell all of their stock and when they challenge Dany she further shows her devotion to buy them all. Some of the Good Master see this am opportunity to earn even more gold.
Dany frowned in annoyance. "Very well. Tell them I will pay double, so long as I get them all."
"Double?" The fat one in the gold fringe all but drooled.
"This little whore is a fool, truly," said Khaznys mo Nakloz. "Ask her for triple, I say. She is desperate enough to pay. Ask for ten times the price of every slave, yes."
When the deal comes down to brass tacks we find that:
- The goods in her ships are worth 1,000 Unsullied.
- The ships themselves are worth 2,000 Unsullied
- Daenerys' crown is worth 100 Unsullied.
"You know the answer to that better than I, Good Master," Dany replied. "Your men have gone through my ships and tallied every bead of amber and jar of saffron. How much do I have?"
"Sufficient to be buying one of thousands," the Good Master said, with a contemptuous smile. "Yet you are paying double, you are saying. Five centuries, then, is all you buy."
"Your pretty crown might buy another century," said the fat one in Valyrian. "Your crown of the three dragons."
"Three good ships should be worth more than a few paltry eunuchs."
The fat Grazdan turned to the others. They conferred in low voices once again. "Two of the thousands," the one with the spiked beard said when he turned back. "It is too much, but the Good Masters are being generous and your need is being great."
This is when Daenerys offers one of the dragons. She knows it is the only way she can get all off them.
Two thousand would never serve for what she meant to do. I must have them all. Dany knew what she must do now, though the taste of it was so bitter that even the persimmon wine could not cleanse it from her month. She had considered long and hard and found no other way. It is my only choice. "Give me all," she said, "and you may have a dragon."
There was the sound of indrawn breath from Jhiqui beside her. Kraznys smiled at his fellows. "Did I not tell you? Anything, she would give us."
And by all Daenerys did mean ALL, and she was given ALL for the dragon:
Her stomach roiling, Dany wheeled her silver about and trotted toward the center of the plaza, and the army she had bought so dear. Rank on rank on rank they stood, her stone halfmen with their hearts of brick; eight thousand and six hundred in the spiked bronze caps of fully trained Unsullied, and five thousand odd behind them, bareheaded, yet armed with spears and shortswords. The ones farthest to the back were only boys, she saw, but they stood as straight and still as all the rest.
So we the total number of Unsullied she purchased was at least 13,600.
The only other example I have found of someone buying another person comes from The World of Ice and Fire where Aegons buys a woman for 7 seven gold pieces.
While riding near Fairmarket in 155, Aegon's horse threw a shoe, and when he sought out the local smith, he came to notice the man's young wife. He went on to buy her for seven gold dragons.