On short regional hops. Budget carriers in Southeast Asia (AirAsia, Scoot, VietJet) and Europe (Ryanair, Wizz Air, easyJet) price one-ways at or near half the round-trip fare, so there is no penalty for buying them standalone. That logic does not carry over to full-service long-haul.
On short regional hops, and this is the real exception to the rule that one-ways cost more. Budget carriers in Southeast Asia (AirAsia, Scoot, VietJet) and in Europe (Ryanair, Wizz Air, easyJet) price a one-way at or near half the round-trip fare. There is no penalty for buying them standalone. Bangkok to Siem Reap on AirAsia for $45 is perfectly sensible as a separate ticket.
That logic does not carry over to full-service long-haul. Sydney to Los Angeles bought as a one-way on a full-service carrier prices brutally, and a five-leg itinerary built entirely from one-ways typically runs 30 to 60% more than the same routing built as a custom multi-stop fare. On a $4,000 base itinerary that is $1,200 to $2,400 extra.
Which is why most good itineraries are hybrids: a fare-constructed ticket carrying the expensive intercontinental legs, plus budget carrier one-ways for moving around cheaply inside each region.