Sounds like folks are working on it: https://en.wikipedia.org/wiki/Automotive_thermoelectric_generator https://en.wikipedia.org/wiki/Automotive_thermoelectric_

Sounds like folks are working on it: https://en.wikipedia.org/wiki/Automotive_thermoelectric_generator https://en.wikipedia.org/wiki/Automotive_thermoelectric_gene... The numbers being floated about seem to be 4-5% fuel efficiency gain. Significant, but as you say it depends on the price.