Your question assumes a static economic situation, which is not what will happen. Launching whole factories to process the raw asteroid rock will cost too much at first. What you want to do instead is launch a starter kit of production machines. You use those machines to make parts for *more* machines out of some of the asteroid, plus some parts you bring from Earth. As your collection of machines in space grows, you can make a wider range of items, and need less from Earth.
In addition to growing the factory, you devote part of the output to products for sale. You start with the easiest products, like water and carbon compounds for fuel, because most anything in space requires some fuel, and they can be extracted by simple heating. You convert H2O + C --> O2 + Hydrocarbons, which is common rocket fuel mix. As you are able, you can start to make higher value items, like spacecraft parts.
Along this path, you may find products and materials you can deliver to Earth for less than the ground-produced version. A first such item would be "pristine meteorite samples". Collectors, jewelers, and scientists will pay large amounts for them, and they don't require any processing, just delivery. Precious metals are often mentioned, but they occur in ratios of 30-100 parts per million in asteroids, meaning you have to do a *lot* of processing to get them out. It may not be worth the trouble until much later in the industrial development. But whenever your cost gets below the competing Earth cost, then go ahead and do it.