Some people don't ever vote because they haven't thought it through enough to see how it affects their actual lives. Even if they understand the implications and intend to, when they're tired, broke, or the weather's bad etc..., they may skip it and think next election they will, or that one time they don't, it won't matter. Giving people a little more reason to push on through a bad day or two and make it to the polls or return they're early ballot on time would make a significant difference. The problem is that most people setting election policy will oppose any effort to get more people to vote, much less providing a clear economic incentive to do so. Would it really be that expensive, no, but they'll fight it with everything they've got.