I would recommend low power nRF51 chips. It's a Cortex M0 with integrated 2,4GHz radio. You can use is for Bluetooth Low Energy and communicate with other chips / smartphones / etc (manufacturer provides a BLE stack) or your own simple protocol. There are some modules below $10, but those would require additional USB chip (like ft232 or $2 pl2303 / ch341 module).

If you need a long distance (like 1km) communication then you need a "larger" radio which works in ISM band. For the cheapest solution I'd go with mass produced devices - two WiFi APs with 2 directional antennas + some custom boards ob both sides or APs with open firmware like openWRT.