When using your professions to make gold you have many options in getting the materials. You can either farm the items needed for it or buy the items needed for it. When it comes down to it for those things you have to compare the costs of what buying and farming would be.
Many people like to say farmed materials are free so they are cheaper. They might be right depending on how they where farmed. There is high cost farming and low cost farming but there is always a cost no matter how you look at it. I personally love low cost farming and do it often.
Low Cost Farming:
The Dungeon Run:
Some people say this is high cost. If you are doing it yourself it is. If you are doing it for someone else it isn't. I often help friends out when they are power leveling by running them through dungeons. All I ask is that I am allowed to keep anything green or better they do not need for their character and all cloth gathered. The reason I call this low cost farming is that I would run them for free just to help them out. I choose, by my own free will, to give some of my time to help my friends. With that in mind, my only expense is any repair bills and travel bills that might arise. Hence, low cost farming.
High Cost Farming:
The Dungeon Run:
When you enter a dungeon for the express purpose of farming materials. Then your time, even if it is of your own free will, is being spent trying to make money as opposed to the previous example where your time is being spent to help another person. Being your real intent is to make money and only that, you are now spending your most valuable resource in an effort to make money. Time!
Now lets compare the two of these. Both would roughly the same amount of time invested and out of pocket expenses but one is done with the expressed purpose to make money. Being you are trying to make money you need to look and see if the money you made was worth it.
Lets say during a run you got eight items to disenchant and three stacks of cloth for 20 minutes of your time then you need at what they will be turned into for you. Lets say all that works out to be two bags you can sell for 40 gold each that means you "made" 80 gold for your time invested which was 20 minutes. (not adding the repair and travel costs and travel time that is)
Looking at the auction house you can see what those enchanting materials would cost and what that cloth would cost. If it is less then 1/2 of the cost you sold the bag for then you "spent" more money farming the materials then it would have cost to buy them.
Don't understand that? I will try my best to explain.
If the cost of the materials is cheap enough, you could have spent 80 on the materials and turned it into a finished product that sold for a total of 160 gold on the market, thus making the same 80 gold you just made.
The difference is in what you spent to get them. You spent travel time, run time, gathering time, return time and crafting time not to mention a small repair bill most likely. Being none of that was factored into the 80 gold profit that means the 80 gold "profit" you made from farming made less money and took more time then the 80 gold "profit" you made from buying the good from the market and crafting them only.
That is one of those times where it is better to buy your materials then to farm them.
If that is the case then why should I ever farm items if buying them is cheaper?
That is not always the case. Sometimes the cost of frostweave alone is more then the cost of what frostweave bags sell for. In a case like that you would not want to buy the items on the market because it will cost you more to do so.
What is the most effective thing to farm for?
This is server specific I am sure but over all I would think Epic Gems are the best item to farm for on any server at any time. On my server, even on a bad day Epic Gems sell for over 100 gold. At high demand times I can see them up in the 300 gold range from time to time.
Low Cost Farming:
The Daily Dungeon:
You do this every day for the frost badges anyway. So roughly every three days you will also have 10 triumph badges. Trade them down and get whichever Epic Gem is selling for the most.
The Daily Battleground:
You do this every day for the arena points anyway. So roughly every few days you can buy one Epic Gem from honor. Trade them in for whichever Epic Gem is selling for the most.
You only have to run it once a week to get enough honor to trade in. If you are a PvPer you will be doing it anyway, if you are not, well, then move this to high cost.
High Cost Farming:
Random Dungeon Finder:
If you are a tank or a healer you can pound out enough heroics to shower yourself in gems all day long for a year in a matter of no time. Not to mention you make 13 gold for each one you do as well. This is a money maker for sure. If you are willing to spend your most valuable asset. Time!
Random Battleground Finder:
Chain running these can get you honor quickly, but no so quick that is it unnoticeable. It will take up lots of time which will start running into your profit. This is a better option, time wise, for a DPS then it is a healer but still not a huge money maker.
Whenever you look at something look at the big picture. Is farming this going to be cheaper then buying it? Is farming this going to give me anything else of use? Sometimes farming is cheaper but never is farming free. It always takes time.
I like to consider leveling a gathering profession to be "free" because you have to do it anyway but in a way, it isn't. Have you ever spent 10 minutes fighting 3 mobs off an herb you needed for a skill up? Well, even if did it to get that skill up point more then that herb you sure as hell paid for that herb. You paid whatever you consider 10 minutes of your time to be worth.
Are there any times when gathering is free because you make it seem like there isn't any good thing to farm?
Yes, there are many times when gathering is free. The time expenditure put into it is so minimal that is can not even be considered a cost at all. Gathering while on your daily frost run? That is free material because you are there for a different reason and just happened to come across the item. Killing mobs for a quest and they land on something, skinning them or picking the herb or mining the ore is minimal time expenditure so it is not even worth counting. You where there anyway.
Any time you are somewhere anyway for another reason, it is free material. I like to pick herbs outside of ICC while waiting for everyone to make it there. I consider that free because if I were not flying and picking herbs I would be standing there waiting. So it is wasted time anyway. I just put the wasted time to use.
Any time you head out for the sole purpose of gathering something. That is time spent which means it is already cutting into your profit if the same thing you are out gathering could have been purchased from the AH cheap enough to not have to go gather it.