The following are some changes that can be made to the design of this sensor to help lower the cost:
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* Rather than using 3.5mm jacks, the sensor cable can be soldered directly onto the Arduino shield.
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* Less expensive cables can be found. Cables with fewer conductors (only three are needed presently) and unshielded cables are both possibilities that might prove less expensive. Also, it may be possible to re-cycle existing wire from broke/out of use equipment.