How to Get Maintain Cheap Goods
You don’t really want to have cheap goods, it should be well balanced to avarage market prices. As you already mentioned by yourself you have to aim for making a building profitable. If resource costs are too high then import additional stuff to lower resource cost price or produce more of whatever you need.
When you have fixed your resource costs you need to check on the price of the good you are producing. If the building is still not profitable after you fixed resource costs, you need to manipulate the price of the produced good. To rise the price you can export it to other nations or you can upgrade other buildings that use the same good as a resource to make sure they will be using more of it.
But be careful because if you are using the same good you produce also as a resource somewhere else, the production costs there will also raise and then you have to do the same again. As mostly everything depends on each other it is a never ending story of rinse and repeat manipulating market prices to assure buildings are profitable.