Apparently, the PSN Store has been updated with the coin purchases and it's not looking very nice.
Deal is, you get your initial FOB for free but 3 others must be bought using Mother Base Coins. They cannot be earned and seems like it will cost ~5000 MBC to unlock 4/4 available FOBs.
Here is a link to interview with Robert Peeler talking about micro-transactions (spoiler free discussion):
Mother Base Coins cannot be earned in-game: