Even in the fantastical world of Dungeons and Dragons, items cost money.  For purposes of understanding the value of gold, silver, and bronze, it will help if you think about the US Dollar.  A copper piece is like a penny, a silver like a dime, and a gold like a dollar.

10 Copper = 1 Silver
10 Silver = 1 Gold

I have created a spreadsheet to help with tracking purchases, Click here and find your tab.  Let me know if you need any help with the spreadsheet.