royal challenge stamina cost.

challenging royal heroes cost me 5 stamina per attempt.
other players in my guild say it cost them 5 stamina once per hero.
has this changed recently?
or, is the repeated cost only for higher powered heroes, like general mulan the substitute milestone?

p.s.~ i would personally check by trial and error, but i have no stamina.

It's a 5 stam cost whether you win, lose or recruit the hero or not.