All skills are profitable if you take the time to do them in a way that gives you more gp. But sometimes you just have to put time into the equation and see that some ways are probably not worth your time. If your good at making alot of money in a certain way like merchanting or monster drops then spending gp on bars to smith into plates for faster smithing exp might actually end up saving you more gp in the long run because it gives you more time to spend on the other things you are better at making gp with. I personally like stringing longbows to make my money and merchanting on the side, so they I can turn around and spend a little gp on getting my smithing level up. As long as you work out what works for you and what you are good at with making gp you can find a balance for what you can afford to spend gp on getting xp and what other ways you can get xp for no cost or even profit.