Rashi states that the slave market we are referring to is the market for an eved ivri – a Jewish slave. The Ketzos explains that a Jew cannot be estimated as a non Jewish slave, since he would never be one. The Rosh, however, says the market is for non Jewish slaves. Rashi’s opinion is difficult, as Jewish slaves are only sold for six years, and therefore the difference in value will not accurately reflect the damage done. Rabbi Akiva Eiger says that even if we were to continually reevaluate the damages every six years (to reflect the ongoing loss), this would be unfair to the damager, since the ultimate sum will be much larger than the one time loss to a permanent non Jewish slave. The Maharshal suggests that Rashi agrees that the slave market used for estimation is that for non Jewish slaves, but that Rashi here is simply giving a rationale for applying such an estimation to a free man. Since a free man can sell himself as a slave, this indicates a monetary loss ascribable to the damage done to his body. See Ketzos 420:1 for more detail on Rashi’s opinion.
The Gr”a states that the verse itself hints to the monetary punishment. The word Ayin is three letters – ayin, yud, nun. If we take the letters after each of those letters, we have the letters pei, kaf, samech. Rearranging those letters spells kesef – money. The verse tells us that for the eye, the damager pays tachas ayin – the letters below (after) ayin.