The modern world, as depicted by Wordsworth in “The World Is Too Much With Us,” may be increasingly prosperous, and it may provide us with more goods, more material objects to buy. But it is spiritually enervating, ultimately of no benefit to the soul. All this “getting and spending” may make us feel a little better for a short time—think of "retail therapy"—but ultimately, it brings us nothing more than a “sordid boon.”

What Wordsworth means by this is that the material benefits we derive from living in the modern world cannot compensate for the damage that rampant materialism has upon our souls. Material wealth may be a boon, but it is a sordid boon, utterly immoral, as it involves giving our hearts away to Mammon, wealth as an idol to be worshipped like a god.

Not only does this sordid boon separate us from ourselves, from our God-created souls, it also alienates us from Nature, which, to an arch-Romantic like Wordsworth, is nothing short of criminal.

We are an intrinsic part of the natural world, and yet, in our unhealthy obsession with acquiring money and material goods, we lose touch with that world, standing apart from the sea, the winds, the sleeping flowers, and all the other features of Nature.

In this short poem, Wordsworth is lamenting the extent to which the modern human has fallen out of step with nature and its beauty. He describes the fact that people today are too used to "getting and spending" and have become, in effect, disillusioned with the beauty of the world around them. They see it as simply something which is their due, something which is always there: "too much with us." According to Wordsworth, we have given away our hearts, and this is a "sordid boon."

Breaking down this phrase, Wordsworth is saying that the bargain we have struck (the benefit, or boon) is an immoral, disreputable, or dirty one. To have given our hearts away has benefitted us very little indeed. We have become too materialistic, too focused on getting things and spending money. But the outcome of this has been that we do not relate to nature or see anything in it that belongs to us. We are no longer in tune with nature; we do not recognize how wonderful a prize it actually is, and we are no longer moved by its beauty.

Wordsworth implies that as Christians, this is not the way we should feel about the world. He reflects that if this is the way Christian people behave towards nature, he would rather be a "Pagan" adhering to an old-fashioned and outdated religion (or "creed"), because at least then he would feel moved by the sea as the birthplace of Triton. At least in those days, people revered the natural world as the home of their gods.

"We have given our hearts away, a sordid boon!"

The sonnet is obviously saying that most of us, or many of us, including the speaker himself, presumably Wordsworth, have become so involved in earning and spending money that we have lost touch with all the beauty of nature surrounding us. He probably had to use the word "boon" for a practical purpose. He had to find a word that rhymed with "soon" and would rhyme with "moon" and "tune," because of the requirements of the type of sonnet he was composing.

What he means by "a sordid boon" is that we have made a very bad bargain. We have given our hearts away in exchange for money. This is indeed a bad or sordid bargain if we have done so. If we have lost our hearts in the bargain, either literally or figuratively, our lives are dark and empty. We are like the blind. Or worse than that: we are like the dead. We are surrounded by beauty and drama but miss almost all of it except for occasional "glimpses" when we are not thinking about practical, selfish, worldly matters. Our minds are filled with an endless stream of consciousness like that depicted by James Joyce in his novel Ulysses and by William Faulkner in his novel The Sound and the Fury.

To understand what Wordsworth means by a sordid boon, we must consider the first part of that same line. We have given our hearts away. That is a horrifying thought. What have we gotten in exchange? What is the boon? Where is the quid pro quo? There is nothing we could receive in exchange that would be worth a fraction of the value of what we have given away. And once we have made a bargain, it is almost impossible to undo it. 

The following line appears in William Wordsworth's poem "The World is Too Much With Us":

We have given our hearts away, a sordid boon!

Sordid refers to something which is dirty, vile, or selfish. Boon is something that is beneficial, helpful, or considered a blessing.

The use of the two in conjunction form an oxymoron. (An oxymoron is "a figure of speech in which two contradictory words or phrases are combined to produce a rhetorical effect by means of a concise paradox." The definition is taken from eNotes.)

The use of the words sordid boon create the same effect which Wordsworth uplifts in the poem itself--one of nature and man contrasted against each other. When mankind focuses too much energy on possessing things, they fail to see Nature in all of its beauty.

Therefore, the "sordid boom" is the fact that mankind is constantly involved in a game of "tug-of-war" (depicted by the oxymoron and contrast derived from the words) with the quest for possessions and the appreciation of nature. Given that there is "little we see in Nature that is ours," man (in a search for materialistic wealth) must look away from nature for possessions.