Upon logging in after reaching a certain point in the story, you're greeted with the chance to expand Mother Base with "FOBs," (forward operating bases) which are basically extensions that grant you more staff members and materials. It was rumored before launch that this mode would be entirely closed off and sold as a paid microtransaction, but that's only partially true.
After booting up the live FOB menu, I saw that the first forward operating base (either oostende, zee brugge, or nokke) was free, with a cost of 0 Mother Base (MB) Coins. After that, the menu opened up with more options ranging from 1000 MB to 1200 MB in different locations. Coin packs range from 100 to 6000 MB for real money.