"Pricing is based on device minutes, which are determined by the duration of tests on each selected device. AWS Device Farm comes with a free trial of 250 device minutes. After that, customers are charged $0.17 per device minute. As your testing needs grow, you can opt for an unmetered testing plan, which allows unlimited testing for a flat monthly fee of $250 per device."
From the FAQ[1] which albeit was a bit buried.
edit: It is also at the bottom of the landing page now.