How much does a 50GB plan cost in full?

There is no in it's entirety. You pay $.99/month EVERY month until you cancel the plan. Be advised that any of the increased iCloud storage plans permit you to sync more data to the cloud. They do NOT increase the available space on your iPhone.

There's no fixed limit. It's a rental of space on Apple's servers, and continues until cancelled or the payment method runs out of funds.

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