Gemara

(Mishnah): If Reuven damaged Shimon, he is liable for five categories of payment: Nezek, pain, Refu'ah (medical expenses), Sheves (compensation for loss of wages until he can work again), and embarrassment.

To evaluate Nezek, if he blinded his eye, cut off his hand or broke his leg, we take the difference of how much one would have paid in the market to buy Shimon for a slave before and after the damage.

84a (Beraisa - R. Eliezer): "An eye in place of an eye" is literal.

(Rabah): He teaches only that we do not evaluate the victim like a slave.

Objection (Abaye): Will we evaluate him like a free man? There is no value associated with a free man!

A child's hand was severed (by a donkey). Rav Papa bar Shmuel said that they should evaluate the Nezek.

Question: We must evaluate what he would be sold for like a slave!

Rav Papa: Go evaluate what he would be sold for like a slave.

The boy's father: No, that is a disgrace for him! Rather, when he grows older, I will appease him with my money.

An ox chewed the hand of a child. Rava told them to evaluate what he would be sold for to be a slave.

Question (Rabanan): You taught that in Bavel we do not collect anything that requires an evaluation of one's slave value!

Rishonim

The Rif and Rambam (Hilchos Chovel 1:1,2) bring our Mishnah.

Nimukei Yosef and Rashi (83b DH v'Kamah): The wound caused a loss to Shimon, for if he wanted he could have sold himself to be an Eved Ivri.

.Rosh (8:1): We evaluate the difference of how much one would have paid in the market to buy Shimon for an Eved Kena'ani, who is sold permanently. There is no evaluation of an Eved Ivri to be sold permanently. He can sell himself for six years, and when he goes free he can sell himself again, and so on, for all his life (so we cannot fix an amount).

Chidushei Anshei Shem (on Nimukei Yosef): To answer the Rosh's question, Rashi must say that we gauge how long Shimon would have lived, and calculate how much he could have received for selling himself for six years at a time until he dies.

Question (Bach CM 420:17): 'To evaluate him like a slave' connotes like an Eved Kena'ani. Abaye said that we cannot evaluate like a free man, for there is no value associated with a free man! Rashi himself (84b DH Pegam) says that we evaluate Pegam (of a Na'arah who was raped or enticed) by considering how much one would have paid for such a girl for a Shifchah (Kena'anis) before and after!

Answer (Bach): It seems that Rashi himself admits that we evaluate like an Eved Kena'ani. However, since this is disgraceful, like the Gemara says, the victim may opt to be evaluated like an Eved Ivri, who serves for only six years. This is less than the evaluation like an Eved Kena'ani; this benefits the damager.

Gra (420:20): In Bava Basra (13a DH Hachi), Rashi says that there is no evaluation of an Eved Ivri (to be sold permanently), for he can sell himself only for six years.

Note: Perhaps Rashi in Bava Kama merely explains that Nezek is paid because the victim lost money (unlike pain and embarrassment, which we do not collect in Bavel because there was no monetary loss). He agrees that the evaluation is like an Eved Kena'ani, like he said in Bava Basra!

Rosh (4): The evaluation of Nezek is according to his craft. If he threads pearls and his leg was cut off, we evaluate the decrease in value of a slave who can thread pearls if his leg was cut off. This is a small amount. If his arm was cut off, it is a large amount. It seems that we estimate according to one's trade only to increase the payment, e.g. he cut off the arm of a slave who can thread pearls. If he cut off the leg of a slave who can thread pearls, and it is a smaller lose than that of a slave who serves his master and does his commands, so we evaluate like for a slave who cannot thread pearls. Presumably, he should not lose due to his craft and attribute. If he wants, he need not use this trade, and he is like anyone else who can be sold for all kinds of usage.

Prishah (CM 420:17 DH v'Chosav): The Rosh said that the (payment for) loss of a leg for a slave who can thread pearls is small, i.e. compared to the loss of an arm. It is not less than that for a Stam slave. Sometimes it can be greater, e.g. if the master wants that the slave can run errands.

Note: The Prishah's last words connote that there is a particular master. Perhaps he discusses one who cut off a slave's leg, and explains that the payment is greater if his master used him for errands.

Poskim

Shulchan Aruch (CM 420:15): To evaluate Nezek, if he removed a limb or made a wound that will not heal, we take the difference of how much one would have paid in the market to buy him for a slave before and after the damage.

SMA (16): We evaluate even a distinguished person as if he were a slave.

Rema: If he was a craftsman, e.g. he threads pearls and his arm was cut off, we evaluate Nezek according to his craft. If his leg was cut off, which does not harm his craft so much, we evaluate Nezek as if he had no trade.

Yam Shel Shlomo (8:11): The Gemara says that one who deafened someone pays his full value, for a Cheresh (cannot hear what he is commanded, so he) is worthless. If he had a craft, Nezek applies to him. Erchin 2a says that a vow to give the (slave) value of a Cheresh takes effect, i.e. if he had a trade. No other Meforshim distinguish like the Rosh. Some say that Sheves depends on his trade, but no one says so about Nezek. We ignore a person's importance, e.g. the king's son. There is no limit to the value of a great Chacham, but even so, if one deafened him, the Mishnah is lenient and obligates only the value of a slave sold in the market. Surely we consider a Stam slave without a craft. We consider only attributes of his body, e.g. if he is tall, strong, or beautiful, which are visible. We do not evaluate what depends on his craft or knowledge.

Rebuttal (Shach 3): Surely the damage is greater if the slave has a craft!

Gra (22): Surely, we distinguish only to pay more. If it was even to be lenient, why did the Mishnah say that we evaluate like a Stam slave, if this applies only to someone idle (without a craft? Rather, it applies even to one with a craft, if this causes that the injury did not decrease his value so much.)