The 10 and 20 mark coins of Frederick only appeared in 1888, and that makes them a little more valuable than other equivalent Prussian coins. The coin in the picture is selling at Westfälische Auktions Gesellschaft for about $280 US dollars and it is well worth that price. It is a problem-free, very appealing coin that will make a fine addition to any collector's holdings. If there were problems like stains, spots, or scratches, or if the coin had been cleaned, it would be worth substantially less.
Here are the statistics for the 10 and 20 mark coins:
10 MARK: 19 mm diameter, 0.115 troy ounces gold
20 MARK: 22 mm diameter, 0.230 troy ounces gold
A good rule of thumb for evaluating these coins is to mulitply the current value of gold (look on kitco.com) by the corresponding amount of ounces of gold in the coin.
For instance, suppose gold is at $1370 per troy ounce. A 20 mark would be worth 0.230 x 1370 = $315 and a 10 mark would be worth 0.115 x 1370 = $160. This is the base value (BV) of the coin.
Great looking coins can be worth more than this.
worn: (BV) + $0 US dollars approximate catalog value
average circulated: (BV) + $30
well preserved: (BV) + $75
fully uncirculated: (BV) + $100
worn: (BV) + $0
average circulated: (BV) + $0
well preserved: (BV) + $60
fully uncirculated: (BV) + $90
If you were to sell your coin to Westfälische or any other reputable dealer, figure a dealer's margin of 50 to 75 percent. That is, the dealer would pay 50 to 75 percent less than the retail value of the coin.