How much money can a smart thermostat realistically save during a heat wave?

Record-breaking, persistent heat is keeping ACs running around the clock and driving up electric bills across much of the U.S.
Smart thermostats typically cut cooling costs about 10–15% (manufacturers claim 8–26%) — roughly $15–$30/month in a hot month for many households — by reducing unnecessary runtime.
They cost about $100–$300 plus possible installation (rebates can help); worth it sooner for high-cooling homes but not a fix for poor insulation or aging HVAC.