Two theoretical models, the 'economic' and the 'optimal' model, have been proposed to explain how the fitness of prey changes in relation to flight initiation distance (FID): the distance between predator and prey when the prey initiates escape. Both hypotheses assume that the cost for the prey of remaining foraging (=capture by a predator) influences their decision to escape and this cost should decrease as FID increases. Much qualitative research supports this prediction; however, no quantitative estimate of the cost of escape behaviour exists. Here, we used a seminatural experiment on predator-prey interactions using the black field cricket, Teleogryllus commodus, as prey. We quantified the probability of survival of the prey as an ecological proxy of the cost of remaining at the foraging patch, and the FID of crickets at four different starting distances. The shape of the curve for the prey's cost of remaining foraging for the 'economic' model graphically fits the cost of staying in the patch most closely. The shape of the curve of survival of the prey, as proposed by the 'optimal model', is also appropriate, but the equation that best represents this curve is quadratic rather than exponential as suggested by the model mentioned.