Recent quotes:

Animals, like humans, place a higher value on what requires more effort.

We have examined the justification of effort effect in animals and found a pattern similar to the one in humans but we propose a simpler underlying mechanism: contrast between the greater effort and the resulting reward that follows. The contrast model predicts that any relatively aversive event will result in a preference for a reward (or for the signal of a reward) that follows. Much evidence supports this model: Signals for reward are preferred if they are preceded by having to make a greater number of responses, encountering a longer delay, or experiencing the absence of food (when food is presented on other trials). Contrast has also been found when the signals are associated with greater rather than less food restriction. We have also found a shift toward the preference of a food location that requires greater effort to obtain. Analogous effects have been found in humans (both children and adults) using similar procedures.