Basement goshiwon rooms are really cheap — red flag?

There is a reason nobody is taking it. 반지하 is cheap because it is a genuinely worse place to live, and the reasons are specific rather than snobbery.

What semi-basement means in practice:

1. Damp. This is the main one. Korean summers are humid and semi-basements hold that moisture. Mould on walls and behind furniture is common, clothes take days to dry, and bedding can feel damp for weeks at a time.
2. Very little light. The windows sit at pavement level, so you get some daylight and no sun.
3. Insects. Ground level plus damp is the combination that brings them.
4. Flooding risk. Seoul's monsoon season (장마, roughly late June to late July) genuinely floods semi-basements. This is not theoretical — it has caused deaths in Seoul, which is why the city has been actively phasing residential semi-basements out.
5. Noise and dust from the street at head height.

Against 320,000 versus maybe 480,000: 160,000 a month is 1,600,000 over ten months. That is real, and I understand why it is tempting on a student budget.

What I would actually say: if the budget is that tight, the better trade is to move further out rather than lower down. A 400,000 won ground-floor-or-above room 20 minutes further from campus is a much better product than a 320,000 semi-basement next to it. The commute costs you time; the semi-basement costs you your health and your possessions.

If you view it anyway, go during or just after rain, look at the bottom 50cm of the walls and behind the wardrobe, and smell the room before anything else. Damp is obvious to the nose in ten seconds and invisible in photographs.