A mina was the equivalent of nearly one pound of silver, which in Socrates' time, would purchase a small herd of goats or two oxen (or, for the party conscious, one hundred and twenty gallons of domestic wine).
Traditionally, this was considered to be an insultingly small amount of money and, thus, would have incurred the wrath of Socrates' judges. More recent scholarship, however, suggests that this was a hefty sum for anyone of such limited means as Socrates. In his day a mina would have been the equivalent of one hundred days' wages for an artisan! Socrates' offer of 30 minae, then, via the aid of his wealthier friends, would have been astonishing since it was equal to eight and a half years of wages.
Why would Socrates propose a fine at all given
his view that he really deserved a reward (the free
meals suggestion)? For Socrates, the loss
of money, though a punishment in the eyes of the law, was of no real harm
as far as he was concerned.