The ESP8266 uses 200 mA at 3.3V when operating, which is 660 mW. With a duty cycle of 4.5%, that’s 30 mW; in sleep mode it takes a bit under 3 mW. People report success in getting it to average 18 mW. It costs about US$6. Brian Benchoff reports a range of 366 m with the antenna that’s built into the printed circuit board. The IXYS SLMD121H04L solar cell costs US$6.20 in quantity 1 and yields 89 mW in, presumably, full sun; it’s 43 mm x 14 mm (150 μW/mm²). So you could run an ESP8266 off it during the daytime with a fairly reasonable duty cycle, given reasonable power circuits. A 10000 μF 35V TDK aluminum electrolytic capacitor costs US$2.60 and is 22 mm diameter × 52 mm long; it can store, in theory, 6 joules, which is 9 seconds of full-duty-cycle operation.
You could set up a mesh network of solar-powered ESP8266s 250 m apart in order to provide a communication line. You could drop them in trees, on top of smokestacks, on the roofs of industrial buildings, painted black. Each board might cost US$25, so you would need to spend US$100 per kilometer.