Cost-benefit analyses are central to mature decision-making and behavior across a range of contexts. Given debates regarding the nature of infants’ prosociality, we investigated whether 18-month-old infants’ (N = 160) prosocial behavior is impacted by anticipated costs and benefits. Infants participated in a helping task in which they could carry either a heavy or light block across a room to help an experimenter. Infants’ helping behavior was attenuated when the anticipated physical costs were high versus low (Experiment 1), and high-cost helping was enhanced under conditions of increased intrinsic motivational benefits (Experiments 2 and 3). High-cost helping was further predicted by infants’ months of walking experience, presumably because carrying a heavy block across a room is more effortful for less experienced walkers than for more experienced walkers demonstrating that infants subjectively calibrate costs. Thus, infants’ prosocial responding may be guided by a rational decision-making process that weighs and integrates costs and benefits.