No telling what they are yet, but the PSN Store listing for Metal Gear Solid 5 mentions that 'in-game purchases' are an option. Look, down the bottom:
There's no more information on what you could be paying for but it's likely they tie into the online portion of the game. For example, one of the pre-order rewards is a 'Metal Gear Online XP boost'. Konami previously highlighted the option to make money from online purchases in PES so something similar seems likely. Although there has been speculation that the way MGS 5's Motherbase works could be monitised, with players able to pay for faster research and so on.
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