I've never come across a country where ATM fees varied depending on the location of the ATM.
Whilst it's certainly possible that there exist some "private" ATMs in various airports that charge more, but the normal bank-operated ATMs will charge exactly the same fees and give exactly the same exchange rate through-out the country.
Specific to Seouls Incheon airport (ICN), there are multiple ATMs within the airport operated by the major Korean banks (Shinhan and Kookmin, and potentially others), all of which will give you the same exchange rate and charge the same fees as the equivalent ATMs anywhere else in the country.
In many cases the fees WILL vary slightly between banks, especially if your home bank has a relationship with one of the local banks, in which case there may not even be any fees. It's worth checking with your bank before heading to any country to see if they have any such relationships. eg, the Global ATM Alliance allows cardholders from certain banks to use some international ATMs free of charge - although they do not have any ATMs in South Korea so this does not help you in this specific case.
Also keep in mind that not all Korean ATMs will work with "foreign" cards - you need to look for an ATM which is specifically labeled as "Global" and/or has the Maestro/Cirrus logos on it. All of the ATMs at the airport should be fine, but elsewhere it may only be one of the ATMs available at a specific branch that will work with your card. (This was the case last time I was there a few years ago, not sure if it's changed by now or not)
Not all ATMs (including those at the airport!) are open 24 hours, with many "closing" at around 11pm. Some, including the Citibank ATM at the airport, are open 24 hours.