Wow, Dutchman. Nice coin! These old 10 and 20 mark coins from the German State of Prussia (Preussen) bear the likeness of King Wilhelm I. Coins with earlier dates in the series, 1871 through 1873, abbreviate 'mark' as 'm' on the back side. This page applies to all coins between 1871 and 1888, although there is an 1888 coin with King Friedrich III (see this CoinQuest page).
The coin in our picture comes from Manfred Olding Munzenhandlung in Osnabruck, Germany. It is a gorgeous coin in gorgeous condition. Olding's coin may not be fully uncirculated, but it certainly is close. It is difficult, if not impossible, to discern 'almost uncirculated' from 'fully uncirculated' from a picture. You must see the coin in person.
To figure value for these coins, start with the gold melt value, that is, the value due to gold content alone. The coin catalogs give us the information we need:
10 MARK: 0.115 troy ounces gold
20 MARK: 0.230 ounces
Now multiply the gold weight by the current value of gold. According to kitco.com, gold currently spots at $1600 US dollars per troy ounce. (Look it up; gold changes value continuously.) At the time of this writing, the melt values are:
10 MARK: 0.115 x 1600 = $184 US dollars melt value
20 MARK: 0.230 x 1600 = $368
To the melt value add the value due to collector demand. These collector premiums do not go up and down with the gold market, but run approximately as follows for these coins:
worn: add $10 to basic gold value
well preserved: add $50
fully uncirculated: add $100
worn: add $0 to basic gold value
well preserved: add $80
fully uncirculated: add $150
There is one more step to figuring value. Some dates and mint marks carry collector premiums that are much higher than the normal premiums shown above. Check the dates as follows:
1876B: add $2000 for well preserved coins
1876C: add $1500
1878B: add (are you ready?) $80000
1882A: add $4000
1883A: add $2000
1886A: add $3000
1877C: add $2000
These values provide general guidance for the price of these nice coins. If you have one to sell to a coin dealer, the dealer will need a profit margin, so he or she will offer you less than these values. A margin of 10 to 40 percent is typical for a single coin. If you have 100 coins to sell, the margin can be much less of a percentage.
CoinQuest thanks Manfred Olding for use of the nice coin photo.