The good thing about German 5 mark coins between 1951 and 1974 is that they are minted in silver. Other denominations, 1 and 2 marks, and 5 marks after 1974, are made of copper-nickel and carry basically no value.
To find the value of the silver in 5 mark coins between 1951 and 1974, look up the current value of silver (e.g., Kitco.com) and multiply by 0.225. The fraction 0.225 is the number of troy ounces of silver in one coin.
For earlier dates and coins in good condition, you can add a few US dollars to account for collector appeal. Such additions run roughly like this:
add $2 US dollars for worn coins
add $6 for average circulated coins
add $10 for well preserved coins
add $40 for fully uncirculated coins
add $1 for worn and average circulated coins
add $4 for well preserved coins
add $15 for fully uncirculated coins
There are three 'better dates' in this series of coins. If you can find a 1956D or a 1958F 5 mark, these catalog around $150 in well preserved condition. Finally, a 1958G is quite rare, tipping the scales at $2500 well preserved.