It costs a considerable amount of money to protect the environment from being damaged by the various parts of society. This money is used to fund agencies like the EPA, scientific and academic personnel that monitor the environment, and subsidies for people who make use of various environmentally friendly technologies. All of this adds up, of course, but it is (on the whole) a better idea to work to prevent the damage than let environmental damage (in the form of damage to agriculture and fisheries, increased medical costs, lost productivity, etc.) wear away at the economy and at Americans.
In general, it is far cheaper to prevent environmental catastrophes than to try to clean up after them. Disasters like Chernobyl or the BP oil disaster in the Gulf (and many more, less spectacular but no less destructive, cases) can wreck local economies. Further, governments at all levels often end up paying for much of the clean-up.
Even these much publicized disasters are mild compared with what could easily happen. Invasive species often wreak havoc and are essentially impossible to eradicate. (See, for example, the enormous damage done by rabbits in Australia and the long series of failed attempts to exterminate them.) A widespread release of toxins in the soil could eliminate swathes of farmland and depopulate residential areas. (Superfund cleanups of toxic spills and dumps already cost the US over a billion a year more than what is recovered from the perpetrators.) Diseases (like malaria, dengue, Nile fever, and swine and avian flu) often become problems in the US as a result of environmental mismanagement here or elsewhere, and impose large recurring costs on the population and especially on the government.