With more generation fluctuations from the high penetration of renewable energies, dispatching large-scale demand-side resources becomes crucial in maintaining the power system balance between supply-side and demand-side. It puts forth significantly greater data requirements compared with traditional power systems, e.g. explosive data volume and highly frequent data collection between the data aggregators (DAs) and the data service operator (DTSO). Preferred by the DTSO, higher-value data with better data quality and timeliness will bring more benefits to the real-time dispatching of the demand-side resources than lower-value data. Thus, the data should be collected, transmitted, and traded in accordance with their varying values across different DAs. To resolve this issue, this paper proposes an incentive mechanism for electricity data trading between DAs and the DTSO, incorporating a data valuation-aware contract. First, we develop a data valuation model considering the data quality and timeliness. Then, taking into account the diverse value of data across DAs, we design a trading volume-reward bundled contract for DAs. The DTSO provides DAs with varying rewards to obtain higher-value data. Finally, the effectiveness of the proposed mechanism is validated using realistic data.