MD
An engineer friend tells me low fuel state warning sensors would cost about £500 for a pair of tanks. On many light aircraft it might even cost less because the sensor could be fitted to the filler cap.
You'd get an early warning on one wing when the aircraft banks, but the sensor would be fitted so that both lights stay lit at a predetermined safe minimum quantity.
Even if the cost was double or treble that estimate, it still seems a very small price to pay for the additional safety.
On many if not most light aircraft, the pre-flight visual check of quantity of fuel in the tanks tells you no more than the tanks are empty, there's some fuel (quantity unknown) or the tanks are full. The only real safety value on many aircraft is that the pilot then knows the filler cap is properly fitted.