With all due respect Akainu is the one and only Marine ( that is still really active, so no Garp or Sengoku in this ) I'm putting all my stocks in.
I don't see a former Admiral getting above 2B, they're strong but not THAT strong.
However, as the Fleet Admiral, Akainu has a lot of room for growth, especially if he's supposed to be comparable to Prime Sengoku.