Nice coin, Vee. And valuable too!
Your 1905 20 mark gold piece from the old German State of Prussia would sell retail well over $300 US dollars at today's gold price. The coin contains 0.23 troy ounces of gold. Let's say gold is priced at $1400 US dollars per troy ounce. That's 0.23 x $1400 = $322 in gold value alone for the 20 mark coin. Use Kitco.com for current gold price. The value changes every day. Look it up now!
The Prussian coin is so beautiful, and your specimen is in such good shape, it should draw several dollars more than its straight gold content. Read further for more details.
Prussia issued a gold 10 mark coin that looks the same as your 20 mark, only smaller:
10 mark: 19 mm diameter, 0.1152 ounces of gold
20 mark: 23 mm diameter, 0.2305 ounces of gold
A good formula to give an approximate retail price for this coin is to multiply the gold weight by the current price of gold and add a collector premium of $50 US dollars. That would be for an average circulated coin without any problems like nicks, scratches, stains, or cleanings. Uncirculated coins might double the collector premium.
DO NOT CLEAN YOUR COIN. CLEANING RUINS VALUE
At $1400 per ounce, the value with collector premium would be:
10 mark: 0.1152 x 1400 + 50 = $210 approx retail price
20 mark: 0.2305 x 1400 + 50 = $370
If you were selling this coin to a coin dealer, he or she would probably pay around 60 to 75 percent of the retail price. The difference is the dealer's margin, which keeps the dealership solvent.
Just about all the gold coins like this, regardless of date, sell for the same price. However, there are some special dates and mint marks where the $50 collector premium goes up, as follows:
1892: add $400 collector premium to basic gold value
1894: add $600
1895: add $400
1906J: add $100 collector premium to basic gold value
1909J: add $100
Storing rare coins in plastic bags is not a good idea. For well-worn, low-value coins it is ok. But you don't have that. You have a well-preserved, high-value coin. Chemicals in the plastic can easily ruin your coin's value in a few short months.
Coin dealers (look in the yellow pages, or online) carry all sorts of coin storage supplies. These are specially formulated containers that won't mess up your coin. Simple plastic 'flips' are cheap and easy to use. The plastic they are made of contains no softeners or PVC, which are the main culprits in damaging coins. At a minimum, return it to the jewelry box as you found it. If you'd like, contact us and we'll send you a flip by snail mail.