Things to watch out for : OPPORTUNITY COST.
That means, if you are selling something, you'd better damn well sell it for more ISK than you could have made by simply selling the things you used to manufacture it with... or else there's no point in manufacturing at all in the first place.

That's the most critical mistake a rookie manufacture makes : selling below cost.
And the reason he does not go bankrupt is that he usually mines the ore himself, then reprocesses it, and uses those minerals to manufacture.
SURE, if you CANNOT sell the minerals at an acceptable price in the location they are in, then manufacturing something that actually sells at that location would make sense (but only if the effort is less than the effort you'd have to make to haul the minerals somewhere they can sell).
But if you CAN sell the minerals decently, manufacturing and selling something below that "opportunity cost" is STUPID.
-Akita T