On the Archdiocese of Washington's Maybe It's God blog, Msgr. Charles Pope says that the Parable of the Lost Sheep "drips with irony, even absurdity."
I'm not entirely sold on that reading of the parable; I remember Cardinal McCarrick writing about seeing a shepherd rescue a lost sheep during a visit to the Holy Land.
But whatever First Century Jewish shepherds (or First Century Jewish housewives, per the Parable of the Ten Coins) might do, it's clear that First Century Jewish Pharisees would not go searching for a lost tenth of Israel, much less a lost hundredth. Their value system held that sinners aren't worth the time spent eating with them. Jesus, though, explains that saving a single sinner is worth everything to God.
Moreover, Pharisees would surely estimate the sinner-to-righteous ratio to be higher than 1:99, making the return on investment of proclaiming the Kingdom to sinners even greater -- unless, again, you consider each sinner worthless.
If the numbers were reversed, even the least sentimental shepherd would go after the ninety-nine lost sheep. Yet the Pharisees, if they followed their principles, would satisfy themselves with one righteous person who had no need of repentance (and just who will that one righteous person turn out to be?) and leave the ninety-nine sinners lost. Clinging to their one scrap of righteousness, how much joy they miss out on!