Precious metals bullion bars and bullion coins are commodities having intrinsic value. What they are worth is discovered by international commodities markets (gold spot market, silver spot market) and can be calculated exactly on the basis of precious metals content. That is what these pieces trade for, and there is a rather fixed amount they are “priced” at. Prices may vary locally only due to fluctuations in an underlying currency (which itself does not have any intrinsic value other than some governments “word of honor” which may well turn out to be zero) or due to local shortages in physical materials. As recently as October 2011, this has drastically been visible in Greece with bullion coins and bars selling in the streets of Athens at premiums of up to 40% over spot. This clearly demonstrates that physical bullion prices do fluctuate dramatically and that local market imbalances (either due to excessive demand, a sudden surge in demand or some actual supply shortages) immediately affect the actual paid price. In order to buy at a reasonable price, a lot depends on both timing (watch market price over a period of time) and finding sellers charging a fair premium (compare different sellers and their markups which can differ regionally; so you might want to watch prices at home as well as abroad).

How Do I Buy and Sell Physical Gold and Silver?
Do I Have to Pay Sales Tax (or VAT or Similar) on Precious Metals?
Should I store my precious metals locally or overseas?
Can I Earn a Higher Profit With Numismatic Coins?
What Is a Reasonable Premium Over Spot?
Should I Buy Bullion Bars or Bullion Coins?
Should I Buy Bullion Online?
Should I Buy Bullion Over the Phone?
Should I Buy Bullion Using eBay or Amazon Marketplace?
Is Confiscation of Gold a Real Risk?
Are There Any Reporting Requirements for Gold or Silver?
Should I Buy Vaulted Allocated Gold or Silver?