How to budget for 7 days in Korea?

I would say you could make do with RM1,200 (~400,000 won) for ground arrangement - lodging, food, transportation in Seoul and admission tickets.

Warning: 

1. This budget is without flight ticket and shopping budget. 
2. Flight fare will depend on which airline and the date you choose to fly ; season or off season.
3. This itinerary does not cover for places out of Seoul except for Nami Island. 
4. For Busan, Gyeongju, Jeju Island and ski or snowboarding trip, you may need more expenses. 
5. Price may change. 

7 days budget (for one person)

Day 1 -  Subway from Incheon Airport to Seoul = 4,800 won 

             (Optional: Limousine bus 14,000 won )

             Guest house stay 6 nights, dormitory style - 20,000 x 6 = 120,000

             T-Money for 7 days - 20,000 won + 3,000 won (deposit) = 23,000

             Jjimjilbang - 12,000 won

             Breakfast/lunch/dinner - 15,000 won

             Tips: You can get a fulfilling decent meal for 5,000 won and rest your 

             bones at jjimjilbang for a change. ^^

Day 2 -  Gyeongbok palace / Chandeok palace - 3,000 won

             Insadong

             Jongno

             Cheongye Stream

             Gwanghamun

             Breakfast/lunch/dinner - 15,000 won

             Tips: If you visit Chandeok palace, you can just walk to Insadong, 

             Jongno, Cheongye Stream and Gwanghamun

Day 3 -  Namdaemun

             Myeongdong

             Namsan Tower and Cable car - 15,000 won

             Breakfast/lunch/dinner - 15,000 won

             Tips: Visit Namdaemun first early morning and try to go in weekday, 

             then go to Myeongdong and after that walk up the hill to Namsan Cable 

             Car Station

Day 4 -  Everland / Lotte World - 35,000 won

             Breakfast/lunch/dinner - 15,000 won

             Tips: Lotte World is in Seoul while Everland is in Yongin. So, you have 

             to take a shuttle bus to Everland and prepare to go out early from 

             your guesthouse.

Day 5 -  Nami Island - 15,000 won

             Itaewon

             Dongdaemun

             Breakfast/lunch/dinner - 15,000 won

             Tips: Go early in the morning to Nami Island because you need to take 

             an hour and a half train to Gapyeong Station. You could visit Seoul 

             Mosque in Itaewon and after that go to Dongdaemun in late evening 

             because shopping in Dongdaemun preferably be done at night.

Day 6 -  DMZ - 50,000 - 70,000 won (depends on package)

             Yeoido park

             Hangang Cruise - 9,000 won / 63 Building (Sky Art, Sea World, IMAX 

             theater, Wax Museum)

             KBS/MBC tour (walking distance from Yeoido Park)

             Breakfast/lunch/dinner - 15,000 won

             Tips: DMZ is the border between South Korea and North Korea. 

Day 7 -  Final shopping day

             Breakfast/lunch/dinner - 15,000 won

             Subway from Seoul to Incheon Airport  = 4,800 won 

             (Optional: Limousine bus 14,000 won )

Estimated total: RM1,200 ~ 400,000 won

 (You can even lower your budget to less than RM1,000 if you exclude DMZ / 63 building)

Check out walking tour budget by KNTO here.