Not much is known, but testing shows 2 rules.
===When the price in store< price sellable to the NPC at (+0.00%), the ===The demand skyrockets. You can reach 5,000 cars/fac/hr when selling at 15,000( though it's not a great idea)
===When the price in store > price sellable to the NPC at (+0.00%), the === The demand is:
(demand at (+0.00%) at 1 store)/2^(log 1.25(price in store/ price in NPC selling))*retail stores.
In layman's terms, double the stores give double the output, but only a 25% increase gives half the output