What is meant by a piece of money in Matthew chapte 17 verse 27 of the Bible. What does it mean in US dollars?

The relevant verses in the King James translation are :-

Mat 17:24 And when they were come to Capernaum, they that received tribute money came to Peter, and said, Doth not your master pay tribute?

Mat 17:25 He saith, Yes. And when he was come into the house, Jesus prevented him, saying, What thinkest thou, Simon? of whom do the kings of the earth take custom or tribute? of their own children, or of strangers?

Mat 17:26 Peter saith unto him, Of strangers. Jesus saith unto him, Then are the children free.

Mat 17:27 Notwithstanding, lest we should offend them, go thou to the sea, and cast an hook, and take up the fish that first cometh up; and when thou hast opened his mouth, thou shalt find a piece of money: that take, and give unto them for me and thee.

The marginal notes for verse 24 also render the Words 'tribute money' as'didrachma, in value 15 pence.' ( A didrachma coin is a coin worth two drachma ). Because of inflation, the value of '15 pence' is meaningless here. However, in verse 27 the marginal note says the 'piece of money' is "a stater. It is half an ounce of silver."

In today's paper ( April 8th 2008) the price of silver was US$18:18 for a Troy ounce, which makes a 'stater' (or half an ounce of silver) worth US$9:09.

The "NET Bible" translates verse 27 as:-

17:27 But so that we don't offend them, go to the lake and throw out a hook. Take the first fish that comes up, and when you open its mouth, you will find a four drachma coin.48 Take that and give it to them for me and you."

Its marginal note 48 says:-

"The four drachma coin was a stater (στατήρ, stathr), a silver coin worth four drachmas. One drachma was equivalent to one denarius, the standard pay for a day's labor "

This means the 4-drachma coin was equal to the standard pay for 4 days, so this would make the denarius , (which was 1 days standard pay), equal to $2:27 in today's money. There is no conflict between verses 24 and 27, where verse 24 refers to the tribute money as a 'didrachma' , or a 2-drachma coin, but this was for one person, so the cost for both Jesus and Peter in verse 27 was therefore 4 drachmas, or a stater, equal to about US$9:09