How is an exchange price determined?
An exchange price is determined by the amount sold, and the amount bought.
The price of Yews and all items are determined by the Grand Exchange, which means prices change often. The current price of 1 Yew is 487 - 500 Yews at that price is 243,500gp. If you want to check the prices you can go in game to the Grand Exchange or you can use the link below. It takes you straight to the Grand Exchange link on the RuneScape website.
Exchange rate is determined by the supply and demand in the market. The more buyers there are, the higher the price of the currency is set and vice versa. While this is basic, there are more dynamics that influence the buyers and sellers. Economic and political stability are one of the few factors that play a role in setting the buy/sell of the currencies.