Worth a Jew's-eye. According to fable, this expression arose from the custom of torturing Jews to extort money from them. The expedient of King John is well known: He demanded 10,000 marks of a rich Jew of Bristol; the Hebrew resisted the atrocious exaction, but the tyrant ordered him to be brought before him, and that one of his teeth should be tugged out every day till the money was forthcoming. This went on for seven days, when the sufferer gave in, and John jestingly observed, “A Jew's eye may be a quick ransom, but Jew's teeth give the richer harvest.”
Launcelot, in the Merchant of Venice, ii. 5, puns upon this phrase when he says to Jessica:
There will come a Christian by Will be worth a Jewess' eye.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Jew's-eye from Fact Monster: