Basically, if the only reason you are crafting is to make your own exotics/sigil/rune then dont do it. The cost of leveling a crafting profession far outweighs the... ~10-20Silver you would save from just buying the item already crafted. (10-20s is probably generous now days).
To make money off of crafting, you would have to craft hundreds of items. Before there were thousands of players making items, you could make money off of buying raw mats and turning them into exotics. I did this around the second week of release and made enough money to make my exotic set by the time i hit 80. However, even when there were far less people with 400 crafting, i could only make about 20-30s per piece, and this profit dropped to about 10-20s within 5 days of me hitting lvl 400. Now days its around 5s if anything...
It is a nice bonus to be able to turn raw materials you get into rares/exotics to sell for.. most of the time more money then the raw mats. It can be seen as a long term investment, as you will eventually get your money's worth out of it, but for a short term, "is this a good way to get my exotics" scenario, then absolutely not.
Now, if you crafting for levels, skins (though crafted skins are ugly imo), achievements or legendary crafting, then yea, its very useful.
As I mentioned, the items on the TP doesn't just appear from nowhere. The reason you can buy them so cheap is that they're crafted so easily. But I do agree, it's interesting to see how the market is, but that makes sense with a system that evolves solely around overbidding each other. Even so, the mats required for crafting are still cheaper than the weapons themself, albeit by a small amount, so depending on how you use the auction house you could still make quite a profit, albeit a profit on the side. The good thing about the auction house is you don't have to do anything, so you can get your money the regular way while you just wait for a bit of money to roll in on the side. But no, crafting is in itself not a great source of income.