Amazon offers its Prime members the Glentronics Watchdog Water Sensor and Alarm, model no. BWD-HWA, for $11.97 with free shipping. (Non-members can spend $49 or more to qualify for free shipping; otherwise, it costs an extra $6.65.) That's the lowest total price we could find by a buck. It detects leakages only 1/32" deep.
Of note, Home Depot offers it for the same price with pickup. (Some locations may be a few bucks cheaper.)