When you type /shop a window will appear that has at the bottom of it a list of the commodities and equipment on your base. The list should look something similar to this:
The first number (01, 02, 03, etc) is the item's inventory number. Then you have the commodity or equipment's name. The next number is the price that the item will buy AND sell for. The next number is the minimum stock you want the base to retain; the following number is the maximum stock you want it to allow.
If you want to set the price of Silver to 100 credits with a min stock of 10 and a max stock of 100 you would use the following command
/shop price 01 100 10 100
01 being Silver's inventory number, 100 being the price, 10 being the min stock and 100 being the max stock.