Let us practice some math:
A) 10 pieces of healing potion, anti-poison potion and anti-paralysis potion = 30 standard potions
B) 30 standard potions + 20 deterrent potions = 200 gold coins
C) 1 deterrent potion = 7 gold coins --> 20 deterrent potions = 140 gold coins
D) 200 gold coins (B) - 140 gold coins (C) = 60 gold coins for the total 30 other potions.
E) 60 gold coins / 30 potions = 2 gold coins for a standard potion.
Result: Just 2 gold coins for the potions that are high in demand and are more useful than the deterrent potion while the deterrent potion costs 7 gold coins?
I think either the author has not thought about the logic at all when writing this or it was meant to be 500 gold coins in total (12 gold coins for each high quality standard potion).