What cognitive mechanisms explain why people underestimate small recurring costs but overestimate one-time large ones?

"There's a well-documented pattern where consumers are more sensitive to a $200 one-time purchase than a $20/month subscription that costs $240/year. I'm researching the behavioral economics and cognitive psychology underpinning this — specific named effects, studies, or first-hand observations from pricing or product work. Looking for depth, not just "it's the sunk cost fallacy"."