Exerting cognitive control is well known to be accompanied by a subjective effort cost and people are generally biased to avoid it. However, the nature of cognitive control costs is currently unclear. Recent theorizing suggests that the cost of cognitive effort serves as a motivational signal to bias the system away from excessive focusing (i.e. cognitive stability) and towards more cognitive flexibility. We asked whether the effort cost of cognitive stability is higher than that of cognitive flexibility. Specifically, we tested this prediction in the domain of working memory by using (i) a delayed response paradigm that allows us to manipulate demands for stability (distractor resistance) and flexibility (flexible updating) of working memory representations, as well as (ii) a subsequent cognitive effort discounting paradigm that allows us to quantify the subjective effort costs assigned to performing the delayed response paradigm.We show strong evidence, in two different samples (28 and 62 participants respectively) that subjective cost increases as a function of demand. Moreover, we demonstrate that the subjective cost of performing a task requiring cognitive stability (distractor resistance) is higher than that requiring flexible updating, supporting the hypothesis that the subjective effort cost of cognitive stability is higher than that of flexibility.