The prices used are either that of items bought or sold from NPC shops, or a reasonable price of the combined cost of its ingredients. For example, the pastry cost fourteen coins minus the one coin refund for the pot (thirteen coins), and the pie dish would have cost three coins if he had bought them, so the pie shell would have cost him sixteen coins. For him to sell it for forty coins to a citizen of Varrock, isn't unrealistic and means he more than doubles his money.
The Grand Exchange has been tweaked to make it more realistic in a world environment. Now people have to deposit items in a bank (any bank) and receive a note. This note is what is sold through the G.E. and the person who receives the note exchanges it for the item (or arranges for it to be delivered to the bank of their choice and then makes the exchange). This means that the G.E. isn't in charge of storing a large amount of items, rather they are stored across the banking world. People can sell their produce and organise transport accordingly. For example, someone from Al Kharid can sell a thousand coal to someone in Ardougne, without having to transport it to the G.E. and from the G.E. to Ardougne.
Checks also appear in the story and work in the same way as real life checks. Notes are used similar to a check. Both of these save on having to transport coins and goods by Urist and NCP's. Rather it is down to the bank to transport them, who have far more security to do so than NCP's. This saves on the need to be concerned about thieves, highwaymen and such alike. Although realistically, these people would still be trying to loot any valuable goods.
The prices for the ore, as they are NCP prices, are the same wherever he buys them. The prices are; Copper 20 coins, tin 20, iron 17, mithril 162, adamantite 400, runite 5000, coal 45. The runite I made up at 11000 (rough G.E. price) is quite expensive, and whilst 5000 is still expensive, it's more affordable. His selling price per bar is; bronze 50, iron 25, mithril 400, adamantine 750, rune 5500.
Bricks, like in real life, had been invented previously, but were made individually and left in the sun to bake. This meant, that like stone blocks, each brick was individual and had its own nocks and crannies. What Urist invents, is the modern day real life brick, which is mass produced and much cheaper to produce as it's less labour intensive. In fact, the version Urist invents, is the first ever mass produced brick, found in the early 1800's. They have since improved with machinery and are even less labour intensive now than the ones Urist invents. But such machinery would take a lot longer to invent, and most likely would cause such a large scale production of bricks, that Gielinor would have far more bricks than they would know what to do with.
Gunnar's Ground subtly appears in the story, and whilst it could be a quest story spoiler, the quest isn't of huge importance to the stories of Gielinor. But as it is still a quest and still a spoiler, I must forewarn people of the spoiler. There may be other quests that appear as well, and although there are spoilers to these quests, the quests aren't of huge importance. Quests are more likely to be of more importance in future stories.