You said you wish to make and sell mithril collections (6,223 gp) to buy a Santa Hat (100.6 million). But smithing is a money-losing skill. A mithril set needs 13 mithril bars. Or if you would like, 13 mithril ores and 52 coal. (13 mithril ores * 375 gp per mithril ore) + (52 coal * 281 gp per coal) = 4,875 gp + 14,612 gp = 19,487 gp 6,223 gp. Selling mithril bars, mithril ores, or coal are all better money-making than selling mithril complete collections.
Stronger money-making in the Grand Exchange comes in purchasing raw material (raw fish, ore, coal, bars) because lots of people want to buy raw stuff to quickly level up their skills (they understand they will lose money but profit quickly skills XP). Any finished products (cooked fish, mithril sets) will typically be sold for less gp on the Grand Exchange than raw materials, you need to keep this in mind. You can sell 100.6 million gp of mithril bars to buy a Santa Hat, but by selling mithril full sets to the Grand Exchange to purchase a Santa Hat, you would have needed 353.1 million gp of mithril bars to make 100.6 million gp of mithril total sets (a colossal money-losing proposal ).
Whilst smithing will make you money if you use it to smelt metal pubs, smithing loses you money if you apply the skill for hammer and anvil metallic processing. It's not by inherently having level 74 smithing (like Salmoneus) which you are able to find a Santa Hat.
And as far as needing level 60 cooking (Salmoneus' cooking level). Raw food typically sells for more cash on the Grand Exchange than cooked food. People who don't want to fish but who want to rapidly raise their cooking level will knowingly get rid of money buy purchasing raw fish, cooking the fish to get quick cooking XP, and selling the cooked (less gp) fish back into the Grand Exchange.
A fantastic thing to do is to fish, sell raws to the Grand Exchange. Maintain you cooking level at 4. And if you need food to train your combat, fish, sell the uncooked fish into the Grand Exchange and purchase fish. You've got the cooked fish to eat and at precisely the same time wind up with more money if you do it like that. Again, if you would like a Santa Hat, a much better idea is to keep your cooking level at 4, and sell raws. Cooking just loses money. Another person who posted suggested runecrafting for good money. But do not bother with this, runecrafting is a completely useless ability. There are far better ways to make money than runecrafting.
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