You do not cut costs by mining minerals yourself. If you mine one tritanium you could sell it for 10 ISK or use it to build something which you can sell for 15 ISK. What free mineral miners do is sell the item they just built for 8 isk, so you actually make less money than if you outright sold the minerals.
When u mine the minerals and use them for manufacture then your cost is throwing away the opportunity to sell the minerals. I made billions from buying ships 20-30% below the cost of the minerals that go into them and then selling them.