Seoul is truly a massive city, and it has things in nearly every price range available. It’s definitely cheaper than Tokyo, but more expensive than most of the larger cities in China. Bargains can be found in Seoul hotels and hostels, and extreme luxury is also available.
Most of the main attractions are well within reach of nearly any budget so at least once you arrive you can see all the major sights without having to go hungry.
Food and drink can also be found to meet nearly any budget, with an abundance of lower-priced options that are generally aimed at the local workforce. If you go to a place with an English-language menu you’ll often be paying a higher price for it, but still there are cheaper options available even for those who don’t speak Korean.