[Pilsner]

If you are adjacent to the river, you can make a flood alarm for almost nothing.

Water dissolves the uncoated aspirin, the clothes pin closes, and completes the circuit. I'd use a lawn mower battery, and larger alarm, to protect a camp, and the cost would be less than $50. Maybe a bit more if have to buy wire, clothes pins, and aspirin.

[chaosagent]

Yeah, but then you need a WiFi/Cellphone connection back to Headquarters for each of the 50 or so ‘sensors’.