How much should I charge?

Im in a weird situation. Ive been teaching piano for years now and have always charged 25 n hour usually 2 lessons a week. I also teach karate but not privately, in a class, never asked to get paid, was more a favor for my sensei. Well the studio closed n everyones gone so I wanted to start teaching kids classes out of my home or fire hall one day a week. Yes im a blackbelt n I have the credentials to do so. So my question is, a lot of the kids I teach piano want to do karate too. My time is worth 25 n hour for piano so how much should I charge for karate? I dont want to appear to be overcharging but I also want to start making extra money. How much do u think?