GetFinalPriceAttribute(){
   $includetax =General::all()->first()->istaxincluded

   if($includetax) return 1
   return 0.5
}

MyConfig::get('istaxincluded');

function get($param){
   if(isset($this->config['param']){
   return $this->config['param'];
}

use MyConfig;
...
MyConfig::get($parameter);