The main reason we don't use fuel aircraft isn't about cost - some of our electric aircraft way exceed the cost of a fuel equivalent. We use electric because of convenience, noise levels, and complexity. UAV stuff adds a lot of complexity to an otherwise "normal" RC aircraft, and so we don't want to add the complexity of an engine on top of that. We also don't need the noise. However, I am currently working on a fuel-powered UAV - purpose: longevity - I can design a fuel-powered aircraft with up to 2 hours of flight time and it's easier and cheaper than designing that feature into an electric - the power to weight ratio of batteries just isn't there yet.