Why is the subway in Seoul so cheap?

The price for riding a subway system has little to do with the cost of operating it, and more to do with how much the government is prepared to subsidize it and the government's attempts at load balancing different types of public transport.

For example the London Underground has lots of long lines with overloading problems on the center of those lines, and lighter loads further out. So tube fares involving zone 1 are relatively expensive, outer zones are cheaper and buses are cheaper still.

Also I don't know where you got your figures for the London Underground, maybe you were looking at a long journey or at overpriced paper tickets. London Underground fares start at £1.50 (~$2) for an off-peak journey that avoids zone 1.

I think that you just have an unconscious expectation that the subway systems of advanced Western countries must necessarily be better than others. The fact is, that they aren't as essential to civilized life there as some other places and actually get way less love.

For comparison the Moscow subway:

• costs 0.6 USD (https://www.rbth.com/travel/326613-how-to-use-troika-card)
• had free WiFi as far back as 2014
• very fast (going 80km/h underground is legit scary)
• very reliable
• not super new overall, but the old stations and the new ones are gorgeous (the ones build circa 1992-2008 are pretty drab).
• maybe not as spotless as Seoul (can't compare, but South Koreans are super tidy), but certainly clean enough to put NY and Paris to shame, from what I've seen.

Remember that in many places (especially in Western Europe), most users will not use single fare tickets but monthly passes instead.

In Paris for instance, even though the cheapest ticket is 1.49 € ($1.66, ticket t+ bought as a carnet of 10 on a Navigo contactless card, valid for travel on the whole metro network, RER inside Paris, Buses and Trams, no transfers between Metro/RER and Buses/Trams), most users have a monthly Navigo pass, which costs 75.20 € per month for the whole region, but their employer is required to pay 50% of that.

So even if they just do one return trip per weekday, that's 0.94 € ($1.04) per end-to-end trip, including any transfers on all 4 modes of transportation. And you can travel from one extreme of the region to the other for that price, and of course the cost per trip is even lower if you use public transport more often.

In Seoul like in many other Asian cities, there are no monthly passes with unlimited use. The "best deal" in Seoul is the commuter pass which costs 55,000? for 60 rides but is valid for 30 days only, and is valid for the subway only.

If one does only one return trip per weekday, that's 1,375? per trip ($1.19), which is not cheaper than the 1,250? ($1.08) cost using single trip fares, and is only a win if users travel more often or make longer (and more expensive on single trip fares) trips.

So it is easy to dispute the fact that the cost is lower in Seoul than it is in other major cities.

In terms of quality of service, it is probably difficult to compare a relatively recent system (less than 50 years for the oldest part, with much of it a lot younger) with systems that are over a century old.

Part of the difference will be the overall standard of living and wage rates. Comparing wage rates is tricky (mean or median?, accounting for the cost of living, whether to deduct taxes & social security). But as a rough indication, look at this:

The minimum wage in the Republic of Korea is ?8,350 (approximately US$7.20).

The minimum wage in the Federal Republic of Germany is €8.84 (approximately US$9.80).

The minimum wage in the UK is £8.21 (approximately US$10.80).

However, the very helpful link in @ceejayoz's comment strongly suggests that the main reason is that the Seoul system receives a bigger subsidy.

How? Subsidies. The government would prefer people to use public transport instead of cars - and the best way to get them to do this is to make public transport so good that you'll tolerate its downsides. It also makes commuting easier, which can reduce pressures on housing around cities.

The ultimate example of this is Luxembourg. As of this coming summer, all public transport across the whole country becomes free. Having visited Luxembourg last year, I can testify that the cleanliness, quality and punctuality of their trains, trams and buses is better than the UK, France and Italy. Even last year, a return trip to anywhere in Luxembourg, on any bus or train, was only 4 Euros. (And less if you bought a travel card.)

The ultimate example in the opposite direction of course is the US. Public transport in the US receives little or no public subsidies. As a result, public transport is universally dirty, poorly-maintained and overcrowded, so everyone drives everywhere. Naturally this leads to gridlock and a greater need for roadbuilding and maintenance.

The irony which opponents of subsidies rarely spot is that roadbuilding and maintenance always come out of public funds. Effectively the public therefore subsidise cars from their taxes, because drivers do not directly pay for the cost of having accessible roads (except on toll roads). This is often missed when discussing subsidies for public transport. There is much discussion about how privatised railways and buses should pay for themselves, but much less about cars. Many countries do have some form of tax on vehicles, but to my knowledge no country has a direct ring-fenced link between taxation of road users and money spent on roadbuilding.