Here's how much I paid for the prototype of a complete sensor module:

• Module board: $ 45.00 
• Wemos Mini D1 PRO board: $ 7.00 
• Max8815_1 step up converter: $ 13.00 
• Supercapacitor 400F XV3560-2R7407-R Eaton:  $ 13.00 
• Supercapacitor mounting clip: $ 1.00 
• Wifi Antenna 8dBi: $ 5.00 
• Module box Gewiss 44006 150x110x70: $ 12.00 
• Solar Panel 6V 3,5 W: $ 12.00 
• Solar Panel mounting support: $ 11.00 

TOTAL:  $ 119.00 

To this must be added the solar panel box, the sensor box and the cost of the sensor applied, which can range from  $ 5.00 to $ 40.00

(depending on the sensor and the evolution of the system).

So it can be said that a complete sensor module can cost between $ 125.00 and $ 165.00

So if we want to create, for example, a network of 60 sensors capable of covering a portion of forest one kilometer wide and 600 meters long, this application would cost us about $ 9,000.00.

In my opinion it is a  cheap price!

Furthermore, the prices indicated are those of a prototype per unit quantity, so this price can go down considerably if you optimize the design and if you produce the components in large quantities.