This work proposes a simple low-cost Wi-Fi-based sensing system using a passive microwave circuit operating at 2.4 GHz. The system replaces the traditionally used vector network analyzer (VNA) with a Wi-Fi access point with an overall cost of 20 USD. The difference in power levels measured with Material Under Test (MUT) and reference level is equivalent to transmission level |S21|. Experiments are conducted with saline solutions of NaCl concentrations in the range of 0.01-0.35 mol/L. Transmission coefficients in different rooms and setups are recorded and compared. Results from VNA are in good agreement with the Wi-Fi-based system and validate the proposed approach.