How much to charge for piano lessons?

It’s cheap, particularly if you’re good at it. Make it an hour, and raise the price appropriately. Then again, I have no idea what the economic climate is wherever you’re situated, the financial capabilities of your students, etc. I applaud your decision and wish you the best.

40 dollars for a 30 minute lesson.

It depends on what area of the country you live in. Check around and see what others are charging.

My area it is about $45 for a half hour for beginners and more for experienced players. That is at the music school downtown with several pianos and instructors.

If you’re just starting out you might consider pricing a little on the low side until you have a clientele and reputation established. $30 for half an hour seems reasonable to me. Perhaps $45 or $50 for a full hour. If you establish a reputation as a quality instructor then you can start charging more.

I would start out low enough to attract as many customers as possible. Then you could see about raising the prices in the future.
Perhaps $25 to $30 for half an hour, $35 to $40 for an hour.

Are you talking about online piano lessons or offline piano lessons?