They aren't going to sell you DLC then make you pay in game money to use it, If mansions are even added (Which they won't be, It wouldn't work fo obvious reasons) then they will be free for everyone just like the Beach Bum pack.
you don't think they'd add DLC you have to pay for IRL and then in game? look at games like Forza. You buy the car pack and it unlocks the cars and then you buy them in game just like any other car. why wouldn't GTA be similar?
If they do add mansions I want a damn level lock like the tank. I don't want the nicest places in the game overrun by low level riff raff who glitched a billion dollars. I want the mansions to be exclusive to people who actually played the game and leveled up to obtain them.