Networking on white spaces (i.e., locally unused spectrum) relies on active monitoring of spectrum usage. Spectrum databases based on empirical radio propagation models are widely adopted but shown to be error-prone, since they do not account for built environments (e.g., trees and man-made buildings). As an alternative, crowd-sensed radio mapping by mobile clients who acquire local spectrum data and transmit it to a central aggregator (platform) for processing, results in more accurate radio maps. Success of such crowd-sensing systems presumes some incentive mechanisms to attract user participation. In this paper, we assume that the platform who constructs radio environment maps makes one-time offers (the incentive for participation) to users, and collects data from those who accept the offers. We design pricing mechanisms based on expected utility (EU) maximization, where EU captures the tradeoff between radio mapping performance (location and data quality), crowdsensing cost, and uncertainty in offer outcomes (i.e., possible expiration and rejection). Specifically, we consider both sequential offering, where one best price offer is sent to the best user in each round, and batched offering, where a batch of offers is made in each round. For the later, we show that EU is submodular in the discrete domain, and propose a mechanism that first fixes the pricing rule, and selects users based on unconstrained submodular maximization (USM); it then compares different pricing rules to find the best batch of offers in each round. We show that USM-based user selection has provable performance guarantee. Proposed mechanisms are evaluated and compared against utility-maximization-based baseline mechanisms.