The cheapest way to introduce silver and gold into your portfolio is by trading it. You are basically buying and selling at the market price. You choose exactly how much you want to buy and you can that amount almost instantly. For a slight percentage you can gain physical possession of the gold and or silver but then you can no longer trade it.
The world leader in online silver and gold trading is without a doubt http://www.bullionvault.com. They possess tons and tons of Gold and have thousands of members trading nearly 24/7. The beauty of trading gold and silver is that you may do so at any time of the day and night as it trades on all markets.