Very nice coin! Back in 1912 the Netherlands issued 5 and 10 gulden coins in gold, as follows:
5 GULDEN (1912 only): 0.0972 troy ounces gold
10 GULDEN (1911 to 1917): 0.1947 ounces gold
These coins can never be worth less than the value of the gold they contain. For instance, if gold is selling at $1200 US dollars per troy ounce (it changes every day; look it up at kitco.com), then the base value (BV) for a 5 gulden is 0.0972 x 1200 = $116 and for a 10 gulden is 0.1947 x 1200 = $233.
In general collectors are willing to pay more than base gold value for these nice-looking coins. Here are approximate catalog values:
5 AND 10 GULDEN:
average circulated: BV + $10
well preserved: BV + $20
fully uncirculated: BV + $50
Unfortunately, there are some really good-looking fakes of these coins. Beware! The fakes are made of brass and are worth essentially zero.