Accurate task pricing in microtask marketplaces requires substantial effort via trial and error, contributing to a pattern of worker underpayment. In response, we introduce Fair Work, enabling requesters to automatically pay their workers minimum wage by adding a one-line script tag to their task HTML on Amazon Mechanical Turk. Fair Work automatically surveys workers to find out how long the task takes, then aggregates those self-reports and auto-bonuses workers up to a minimum wage if needed. Evaluations demonstrate that the system estimates payments more accurately than requesters and that worker time surveys are close to behaviorally observed time measurements. With this work, we aim to lower the threshold for pro-social work practices in microtask marketplaces.