Base_cost = Base cost of item.  This is the unhacked cost with no tonics.

Tonic_Discount = Sum of all discounts from tonics, per above
  up to (35%) for all three vending tonics

Hacking_Multiplier = 100 if the machine is unhacked.
                   = 80 if the machine is hacked.

Final_Cost = Base_cost * (Hacking_Multiplier / 100) * (100 - Tonic_Discount) / 100