How much a policy is expected to cost and who bears the brunt of that cost play a significant role in the debates that shape regulations. We do not have a good track record of predicting costs and their ultimate distribution, but systematic reviews of past assessments have identified some of the factors that lead to errors. A wide range of expected costs of climate policy have been hotly debated, but all are likely to be wrong. This does not mean that we should continue a debate using ill-informed analyses. On the contrary, we need early small experiments to shed light on key unknowns. Environmental stewardship is a long-term challenge and an adaptive regulatory approach promises to inform policy targets and improve controls through sequential regulatory phases that promote: innovation, flexibility and diffusion of best technologies.