I'd like to know if it is possible to turn a profit by creating weapons and armours and then selling them, or if the item simply is worth the sum of all it's ingredients? Quite a few games I play other than Skyrim are like this.
It is certainly possible, but, by the time you have the necessary level of mercantile skills not to be ripped off, gold will become a non-issue. The best use for smithed goods is to be used as vessels for enchantments, which will turn quite a buck.
There is one exception to the rule, though, and it is smelting dwemer metal junk and turning it into dwemer goods, the price difference is quite great. The pitfall here is that dwemer junk is very heavy and is generally located far away from smelting/smithing facilities.
Weapons enchanted with Banish will give you an incredible boost in value to any weapons you enchant with it.