It all starts with ship insurance.
Ship insurance creates isk out of nothing, Isk Deus Machina, if you will, in return for the minerals destroyed when a ship is blown up, thereby performing a service normally provided by the market (player gives up a product, receives isk). By doing so, insurance sets a price floor (the minimum value of an item) on ships. Why would anyone sell a ship for less than the payout they get when it explodes? Rationally, they wouldn't. Not only does insurance set a price floor on ships, it sets a minimum value for the group of minerals used to produce the ship. For example:
Say a dominix takes 5,000,000 tritanium, 1,000,000 pyerite, 500k mexallon, 250k isogen, 100k noxcium, 50k zydrine, and 10k megacyte. That bundle of minerals will never be worth less that the isk you receive from building a dominix with it and exploding the dominix for insurance money.
The pressures of competition, which are huge in Eve considering the amount of people who build things, always pushes items toward their rock bottom price over time. As it becomes less profitable, people build other things... but as soon as there is a profit to be made, people rush into the business and once again drive down the price.
By creating a constant (the value of a bundle of minerals) which involves seven variables (the value of each individual mineral) you guarantee that when one mineral goes up in price, others go down to compensate. Similarly, when one falls, others rise to compensate.