According to the Joongang Ilbo (HT ROK Drop), your take-home might not be as nice as it’s been the last six years:
Expats earning less than 100 million won ($86,133) a year will likely pay more taxes this year than last year because the government is ending a special tax benefit that foreigners had received.
A temporary tax benefit for foreigners enacted in 2004 had enabled foreigners to choose either to have 30 percent of their annual taxable income exempted from taxation or paying tax on only 15 percent of their total annual income including non-taxable income. Either way, most foreigners paid lower taxes than Koreans earning the same income. The revision in the tax act, however, ruled out the 30 percent tax exemption. A rule of thumb is that getting 30 percent of taxable income exempt is a better option for foreigners earning less than approximately 100 million won a year.
An example introduced by the National Tax Service was that of a 38-year-old foreigner who earns 50 million and pays 1.58 million won in taxes when he chooses the 30 percent exemption. If he chose the 15 percent rule, he would have to pay 7.5 million won.

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This is interesting, not sure if there was a typo, or a misstatement. Most likely I’m probably missing some info, but from the source article…
The top line, just below the headline: “February 20, 2010
Expats earning less than 100 million won ($86,133) a year will likely pay more taxes this year than last year because the government is ending a special tax benefit that foreigners had received. ”
Later in the source article, but not quoted above: “‘It varies from person to person, but it is likely that foreigners with less than 100 million won in income will pay more taxes from this year,’ said an NTS official. …’The 15 percent rule is scheduled to expire in 2012..” The NTS official goes on to say they might extend the tax break…
A thoroughly confusing article though not quite as definite, also not definitely as soon as the date, this post indicates; the source article raises more questions than it answers.