There sure are a lot of coin references circulating around Measure for Measure, wouldn't you say? Throughout the play, coining and minting become metaphors for everything from biological reproduction to Angelo's corruption as the Duke's deputy. Let's take a look at some specific examples so we can see what all this coin business is about.
In the following passage, Angelo compares the making of illegitimate babies to the minting of counterfeit coins ("stamps"):
Ha! fie, these filthy vices! It were as good
To pardon him that hath from nature stolen
A man already made, as to remit
Their saucy sweetness that do coin heaven's image
In stamps that are forbid: (2.4.6)
Today this seems like a strange metaphor, but it was pretty common in Shakespeare's day when children were often described as being stamped or impressed by their fathers' images, much like metal coins were imprinted by images (often of the king) before being put into circulation.
Angelo's weird metaphor makes even more sense when we consider that, in the play, sexual reproduction is a crime punishable by death, just like counterfeiting coins was a capital crime in Shakespeare's England.
The funny thing is, Angelo, whose name is associated with the "angel" or "nobel-angel" (a type of gold coin bearing the image of the archangel Michael) is associated with coins throughout the play.
When the Duke announces that Angelo will be his deputy, Angelo likens himself to a metal coin that should be tested for its value and worth:
Now, good my lord,
Let there be some more test made of my metal,
Before so noble and so great a figure
Be stamp'd upon it. (1.1.2)
In a display of false modesty, Angelo suggests that he has yet to prove that he has the substance or worth (mettle) to be the Duke's deputy. (In sixteenth-century England, "metal" and "mettle" were used interchangeably.) Of course, Angelo's use of the word "metal" conjures up the imagery of coining. When he says there should be some test of his metal, he's making a reference to how, in the sixteenth century, a coin's value was based upon the value of the metal from which it was made.
The idea is that, as the Duke's new deputy, Angelo is like a coin that has been stamped with the Duke's "nobel" image upon it. A few lines earlier, when the Duke wonders how Angelo will represent him, he asks "What figure of us think you he will bear?" (1.1.2). This reaffirms the sense that Angelo is like a coin that bears his image.
We all know what a lousy deputy or representative Angelo turns out to be. When he sentences Claudio to death for the crime of fornication and then, like a hypocrite, turns around and propositions Isabella, it's clear that Angelo is corrupt and doesn't have the kind of substance or "mettle" that a deputy of the Duke should have.
This is why we're not surprised when Escalus accuses Angelo of being like a counterfeit coin:
I am sorry, one so learned and so wise
As you, Lord Angelo, have still appear'd,
Should slip so grossly, both in the heat of blood.
And lack of temper'd judgment afterward. (5.1.14)
When Escalus lights into Angelo for being corrupt on the inside while appearing to be so "learned and wise" on the outside, he uses the language of coinage to describe Angelo's fall from grace. Escalus plays on the word "slip," which literally means "to make a mistake," but it also the name for a counterfeit coin. In other words, Angelo is a complete phony.