Clearly the way in which the title of this poem is "Avarice" suggests that greed is going to have some bearing upon the poem. However, as the poem addresses "Money" from the beginning, and in quite unflattering terms, it is clear that this poem is a sustained attack on the effect that money has on us as human beings, and how, above all, when we pursue money, we only damage and hurt ourselves in the process. The poem is full of irony as it focuses on how we have made money, but through making money have made ourselves less as humans:
Then forcing thee by fire he made thee bright:
Nay, thou hast got the face of man; for we
Have with our stamp and seal tranferr'd our right:
Thou art the man, and man but drosse to thee.
The act of creating and stamping money therefore in some way, according to Herbert, has made us less and money more, as we become "drosse" and money becomes "the man," our rightful position. However, if you are looking for one quote that seems to express the theme of this poem, look no further than the final couplet:
Man calleth thee his wealth, who made thee rich;
And while he diggs out thee, falls into the ditch.
Here you have it in a nutshell: the way we treat money makes money rather than ourselves rich. In the pursuit of wealth, as we metaphorically try to dig it out of the ground, we only serve to make ourselves fall into the "ditch" as avarice takes over.