The level of the hero is irrelevant. A level 15 hero with a highest stat of 45 is worth less than a level 2 hero with a 62 in one stat. In fact you pay for heroes based on their level so a low level hero with high stats is preferred.
When hiring heroes you pay a hiring fee. This convinces them to stop running the countryside looking for fights and convinces them to stay in your city and lead your army. This fee is 1000 x the level of the hero.
When you hire a hero you also pay a salary to keep the hero happy. The salary is based on the level of the hero NOT the stats of the hero. The salary is 20x the level of the hero. So a level 1 hero costs you 20 gold an hour. A level 10 hero costs you 200 gold an hour. Etc.
So low level heroes with high stats are more cost effective