Okay the grail didnt help piggy bank doubling the money it gets but i feels like destroyable thing will more likely to drop money.
the "greatly increases luck" makes the luck increase by 200 instead of 100. so having 2 grail makes me have +400 luck.
the "doubles money earned" doesn't stack and doesn't help servant like my assumption before. Only affect player.
So it does help getting more money but its not that much as we might expect