Maybe they don't cost any money at the checkout, but the cost to the environment and to wildlife is too high to ignore.

Hundreds of thousands of sea turtles, whales, and other marine mammals die each year from eating discarded plastic bags they've mistaken for food. Forest animals suffer from trees being cut down for paper bags, the production of which also creates greenhouse gasses, toxic air pollution and water pollution.

Millions of bags, particularly the plastic ones, end up as litter, where they break down in the environment slowly (it can take hundreds of years) and poisonously. Toxins from decomposing bags seep into soil and waterways, entering the foodchain and watertables. To prevent unsightly litter and these more sinister effects some states, like Rhode Island, for example, spend about a million dollars a year picking up plastic grocery bags. Once they're in the landfill, that's where they'll stay, since it's impossible for either paper or plastic to decompose without light or oxygen.