What does it cost for iPhone back up

You can backup your iPhone for free. Doesn't cost anything. Now, if you need extra storage space on iCloud to back your iPhone up then it costs, for example, $0.99 per month for 50GB. Costs more if you need more than that. You can always backup your iPhone to your computer (Mac or Windows) for free.

Axel F.

Do you mean to do this: How to back up your iPhone, iPad, and iPod touch - Apple Support

It would only cost money if you go the iCloud route and you don't have enough space and have to buy more.

You are welcome.

Axel F.