Adequate coverage is very important for sensor networks to fulfill sensing tasks. In many working environments, it is necessary to make use of mobile sensors to provide the required coverage. We propose to deploy a mix of mobile and static sensors to achieve a balance between sensor coverage and sensor cost. We design two bidding protocols to guide the movement of mobile sensors. In the protocols, static sensors detect coverage holes locally by using Voronoi diagrams, and bid mobile sensors to move. Mobile sensors accept the highest bids and heal the largest holes. Simulation results show that our protocols achieve suitable tradeoff between coverage and sensor cost. I.