Thank you all.
You are right, it's a very vague question. Should've known better. I do apologies.
So, let me try to be more specific.
I'm from Buenos Aires, Argentina. I don't have issues getting neither coal nor propane tanks.
Converting to U.S. Dollars, the cost for both is as follows:
Coal - 1 KG (2.2 pounds) : 0.9 Dollars
Propane Tank - 45 KG tank (tall tank, i'm pretty sure its the 100 pounds tank (23.5 gallons) ) : 50 - 70 Dollars depending on the vendor
Propane Tank - 10 KG tank (small tank, 20 pounds) : 10 - 15 dollars. (Yes, the small tank refill costs a lot less than the big one, don't know why)
What do I want to do and what do I don't want to do with smithing?
Well, let me start saying i'm a hobbyist. I love hobbies and I love crafting arts. I like woodworking and i'm starting to weld as well, and i'm pretty good at it. I've never forge anything, but I like knives and metal weapons in general, although i'm not even close to even try to forge a sword. But I do want to forge knives, both for myself and to give away as presents. I'm not planning to work as a blacksmith and I probably won't forge every day, mainly because I have to divide my time free time between other hobbies, but I'd love to be able to if I'm feeling like doing it.
What do I mean with "efficiency" ?
Well, tricky question.. Although money is very important, since i'm taking this as a possible new hobby, i'm more concerned about time and space. If forging a knife requires 40 kgs (88 pounds) of coal or 40kg of propane (20 gallons or so), propane is more practical, because 1 tank takes less space than 40 kg of coal, and this would still mean only one trip. One trip to get the coal or one trip to get the tank. Even better if a 45kg propane tank lasts more than a single knife.
So I guess my question is, when compared in equal environments (everything else is the same, like, for example, the experience of the blacksmith or the quality of the forge), which forge would consume more when doing the same item, let's say a small simple knife?
I hope this clarifies my doubt. If not, please, let me know.