You might also consider using ESP8266 + relay combinations, instead of Pro Mini + nRF24L01 + relay sets. An Arduino IDE setup is available to support C code on ESP8266 systems (1,2,3).
Note: As a consequence of its higher clock speed and larger RAM size (80 MHz or 160 MHz clock, and 80 KB DRAM, vs Mini's 16 MHz clock and 2 KB SRAM) a typical ESP8266 board uses more power than a Pro Mini + nRF24L01 does. According to nrqm.ca/nrf24l01, the nRF24L01 draws around 11 or 12 mA when sending or receiving, and 22 μA in low-power standby mode. According to wiki.iteadstudio.com/ESP8266_Serial_WIFI_Module, an ESP8266 draws ~ 60 mA receiving and 135–215 mA sending. (That current range covers a power output range of 5.5 dBm. It probably supports a much bigger communication area than nRF24L01 does.) Of course, if all your installations have mains power available, the power requirements might not matter much to you. Note, ESP8266 boards typically run on 3.3 V power instead of 5 V power.
As far as cost goes, at the moment Pro Mini clones cost about $2.15 each. nRF24L01's run about $0.90. ESP8266 modules are $2–$3 depending on model and level of module (1,2). You might save a few dollars on your whole system using ESP8266's, but the smaller-sized electronics they provide probably is more important.