Meeting future water demand without degrading ecosystems is one important indicator of sustainable development. Using simulations, we showed that compared to existing policy, more sustainable water supply options are similar or cheaper in cost. We probabilistically forecasted the Mexico City metropolitan zone population for the year 2015 to be 23.5 million and total required water supply to be 106 m3 s−1. We optimized existing and potential supply sources from aquifers, surface water, treatment/reuse, and efficiency/demand management by cost to meet future supply needs; the applied source supply limits determined the degree of sustainability. In two scenarios to supply 106 m3 s−1, the business-as-usual scenario (zero sustainability) had an average relative unit cost of 1.133; while for the most sustainable scenario (it includes reducing potential supply basins' exploitation limits by 50%), the value was 1.121. One extreme scenario to supply the forecast's 95% confidence value (124 m3 s−1) showed little unit cost change (1.106). The simulation shows sustainable policies can be cost-effective.