Gorgeous coin, Nan. These were minted in the German City of Hamburg and the 20 mark contains 0.230 troy ounces of gold while the 10 mark contains 0.115 ounces. There were also 5 mark coins with similar patterns minted in gold and silver. This page applies to the gold 10 and 20 mark coins.
Assuming a gold price of $1000 US dollars per troy ounce, the gold content alone makes the 20 mark value at least $230 US dollars and the 10 mark $115. To get accurate values you must look up the current value of gold at kitco.com and multiply by 0.230 for 20 mark coins and 0.115 for 10 mark coins. The value of gold changes every day.
The gold value is not the only value for these coins. Add to that a numismatic (coin collector) value of $50 to $100 depending on condition, and you have a nice, valuable coin. The coin in the picture has frosty mint luster. That will put it near the $100 level in added numismatic value. It the coin were damaged (scratched, stained, cleaned) the numismatic value goes to zero, but the gold value stays intact.
All the dates in this series are about equal in value except a few dates which have lower mintages, more demand, and higher prices as shown in the list below. The numismatic added values, over and above the gold value, are, on average:
1876J: add $1000
1908J: add $100
1879J: add $250
1881J: add $8000 or more
1908J: add $10000 or more