Midrange
$$
Pho Bay
7~10,000
next to Lotte Hotel
Pho Bay is a great Vietnamese restaurant. Try the Pho Noodle soup.
$$$
Top end
The second floor of the Samsandong Skyrex apartment building the tall twin buildings that can't be missed hosts a buffet called D'Maris which is focused on seafood, but also hosts a large variety of other dishes and is quite delicious. Cost is 36,300 Won per person.