Costs will never go higher than stores"main inventory" (that I figure is normal) And prices will never go lower than stores"players inventory" ( not sure about  OSRS gold the negative effects of this) Above post might not be 100% accurate but the general idea is there. Here's the data from the KB. What affects the cost of things?

As you buy and sell, you will likely notice that items have a inclination to modify price. This can be based on how much value players set on these items. When an item is hard to get and quite useful, you'll probably see its price increase, as buyers are willing to pay more to make certain that they receive a few, and sellers put the selling price higher than the average.

This can be called the Law of Supply and Demand. When an item is in low demand with higher supply, the price will fall. In case the requirement for a product is matched by its own supply, the price will remain fairly constant.

Since you can normally only bid 5% above or below the average price of an item, you are going to realize that there's little danger of an item's cost payable or skyrocketing too quickly. This makes it tougher for players to manipulate prices and ensures that everyone gets a reasonable deal.

Rates are entirely set by players trading! There's no'set value' that costs are based on, with the exception that a cost will never rise above a shop's key stock price, or drop below the price a shop will pay for it. Note that stores will adjust their prices based on this Grand Exchange, so it's extraordinarily hard to make a quick profit by buying from cheap RS gold the shops and selling to the Grand Exchange and vice versa.