Override Magento 2 JS file via requirejs-config.js not working

placing the file under app/design/frontend/Mytheme/mytheme/Magento_Catalog/web/js/price-box.js should be sufficient (without any require config) to overload the file in your theme.

As already said, try clearing the pub/static/ folder content (except the .htaccess). On unix systems you can do this with the command rm -rf pub/static/* from the root directory since * does not include any files starting with a .

If this does not work, I assume there is a problem, since the original File is located in the base area while your theme is in the frontend area. Logically (if this is not working), since base should be inherited by all areas, this might be a bug in the overloading mechanism.