And here I got Ya
Max ammount of cash which You can spend to buy drugs for small boat/truck depends on game day
Its working exactly by this way :
max 750 000 000$ days 1 - 50
max 2 500 000 000$ days 51 - 100
etc. I won`t say more.
About investment. It is working like it`s working because player have to think when he should make investment and when he shouldn`t.