It seems like the base bet is not based solely on VIP level, but also recent action.
A plat 2 that wagered $6k over the past 6 months is likely going to have a lower base amount than a plat 2 that has been wagering 60k a month for the past 6 months - and if both players wager 0 next month, I would expect the second player to still receive the higher base amount.
You claimed a couple months ago that you don't play at Stake any more. If that's still the case, and you got $12, then $12 is the base amount, which, imo is generous.
That is not what a "base" is, base means that you should be getting that no matter what, doesn't matter how much you gambled, could be 1 dollars, could be a million dollars, there needs to be a "base" amount that you will get one way or another.
If a person with platinum 1 gets 50 bucks, and someone with platinum 2 gets 12 dollars, then maybe the base is 12 for plat 2, and lower for plat 1, and that plat 1 gambled way more than the plat 2 in order to get higher. I still think that it is too small of a number to care, like a guy who wagered 400k got 200 dollars in return, I do not think that they should even care for such small amounts when they are gambling so much.