These are sometimes called Lo-han coins, a name that stems from an old story about Emperor Shen Zu who presumably fashioned some of these coin in gold, making them very valuable and highly sought. Unfortunately, this is only a story and there is no gold.
The four Chinese characters top, bottom, right, left are Kang Xi Tong Bao, sometimes rendered K'ang hsi tong pao. The coins are quite common and typical values are:
worn: $1 US dollar approximate catalog value
average circulated: $5
well preserved: $15
Modern reproductions are usually made in the form of large charms and carry basically no value. Genuine coins from the Qing dynasty usually measure about 25 mm in diameter.
The special coin in our main picture (upper left) is an engraved master from which normal coins are cast. The master is very valuable. This one comes from Heritage Auctions and sold for $2000 US dollars during a 2015 auction. CoinQuest is indebted to Heritage for use of their coin photo.
The two characters on the back of the coin are called boo chiowan, and these appear on most coins. Sometimes there are different characters on the back, which generally increases value by a factor of two or three.