Sigismund (Zygmunt) III, king of Poland between 1587 and 1632, must have really enjoyed seeing his name and face on coins. He minted tons of them, each with slightly different portraits and inscriptions, which makes identification tough. They are usually pretty well worn, so that doubles the difficulty when reading the already-difficult inscriptions. Then, just when you think you have faithfully ID'd your coin, the catalogs chime in with phrases like varieties exist.
Richard's coin has a few solidifying characteristics. The SIGIS III inscription helps narrow the playing field, since many coins have SIG III, SIGISMVND III, and other variations. The GROS ARGE inscription pretty well identifies the denomination as silver groschen (or groszy, or grossus, or szostak), which usually come in multiples of 3, e.g., 3, 6 18, 30. Even with all this so-called 'analysis,' our identification of Richard's coin is little more than a guess.
Assuming we got it right, Richard has a 6 groschen from old-time Poland, minted between 1623 and 1627. These catalog as follows:
worn: $25 US dollars approximate catalog value
average circulated: $90
well preserved: $250
coins with a longer reverse inscription of 'GROS.ARGEN.SEX.REG.POLONI' and no date are slightly more valuable and catalog at $150 average circulated
For an idea of price variation with wear, consider the two coins in the secondary image. One is well worn and sold on eBay for $7.50. The other is well preserved and sold in a 2012 Spink auction for $100.