Before you go get your Google Play card(s), download the Android app, and go through the process of the premium upgrade, at least to the point just before you actually pay. It will tell you what the total is that you need on the card(s). The reason I suggest this is that, on once you get a Google Play card and add it to your account, it's "stuck" there - you can't change your mind. Once you buy the Google Play card and add it to your account, you have to spend the credit on things in the Google Play ecosystem, there's no way to extract the money back out in cash again. So it's best to know what the total is going to be before you commit.
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