I know that some people are instinctively repulsed by such a model, but there are good reasons why users prefer it. There is real value in being able to install an app, see that it actually runs on your device, explore the interface and perhaps try some of the features before actually paying anything. Most app stores do have refund policies that amount to a free trial, but there's a big psychological difference between trying an app for free and paying for an app that you can get a full refund on (subject to terms and conditions).