To make sure your money ultimately goes to worthwhile projects, look for certifications by auditors or standards groups like The Gold Standard or Green-e. Companies and nonprofits that deal in carbon offsets will list these certifications on their websites if they have them. Some standards groups also have databases of projects that have been verified by third-party auditors.

There is no fixed price on carbon, and the cost of an offset varies from project to project, depending on how expensive it is to run a given program, said Sarah Leugers, director of communications at The Gold Standard. In general, though, carbon offsets probably cost less than you think.

Consider a trip from New York to Los Angeles. Flying 2,500 miles in economy class will burn about 0.29 metric tons of carbon per passenger, according to the International Civil Aviation Organization’s carbon emissions calculator. And it will cost about $3.26 to offset the approximately six hour, one-way flight using the travel offset calculator by Cool Effect, a nonprofit organization. If you round up to a full ton of carbon, you’d still only spend $3.30 to $13.18 on the Cool Effect website, depending on the project.

Not all carbon offset programs are created equal, as made evident in a recent investigation of forestry projects by ProPublica. To shop smarter, check to see how much money goes to the organization’s overhead rather than to the project you want to support. And, keep an eye out for projects with collateral benefits, like contributing to sustainable development, improving air quality, or strengthening biodiversity.

If you’re really worried about the carbon impact of your travel, the most effective thing to do is avoid flying as much as possible. But, when that’s not realistic for you, carbon offsets can be a relatively inexpensive way to mitigate the damage.