The need of large-scale monitoring of power quality (PQ) in electrical power systems require the design of reliable and cost-effective measurement instruments. In this framework, Field Programmable Gate Array (FPGA) architectures seem to offer valid solutions since they allow powerful measurement algorithms to be implemented on low-cost hardware with reconfigurable firmware. This paper proposes a FPGA-based measurement instrument for harmonic and interharmonic monitoring in compliance with the most important PQ standards. Key feature of the proposed instrument is the very low requirements of hardware and firmware FPGA resources, thus making it suitable for applications that require cost-effective measurements. The proposed instrument shows performance comparable with one of the powerful and more expensive measurement solutions available on the market. In particular, despite the absence of a sampling frequency synchronization system, the proposed instrument proves to be compliant with IEC 61000-4-30, even in the presence of huge frequency deviations. After a description of the proposed hardware and firmware solutions, the experimental characterization of the proposed instrument in emulated and real environments is reported.