Stamina purchases using Gold

Does anyone know the breakdown of how much the cost is each time you purchase stamina using gold? If my numbers are incorrect please let me know.

Thanks

Example: VIP Lvl 12

1st purchase: 140 Stamina = 40 Gold
2nd purchase: 160 Stamina = 60 Gold
3rd purchase: 160 Stamina = 60 Gold
4th purchase: 160 Stamina = 60 Gold
5th purchase: 160 Stamina = 100 Gold
6th purchase: 160 Stamina = ??? Gold