Annual price of precious metals
You have a few options for data. With questions of this nature, Quandl is your friend.


See here for historical gold prices, 1833 until present. I've not read about the methodology here though.
This resource has the gold/silver price ratio. Annual data. It seems pretty easy to transform this into price of silver.
For platinum
It also looks like the Perth Mint keeps a decent amount of historical data. I noticed in that list paladium as well.
The Reserve Bank of Scotland has a set which can give you prices of commodities, monthly and daily going back 15 years. I am not sure if there are any precious metals in that list.