Go through your results for flights to Marathon in the USA with Netflights.
If these flights are too expensive, try searching for a different time of year. So if your dates are flexible, consider looking at different times of year.
Or broaden your search by looking at other airports in the USA.
You can also broaden your search by looking at more than one departure airport. Have a look around – you might be able to save by flying from a different airport or city.