The way the system works is a block reward is paid every 10 minutes, currently 6.25 bitcoin worth about US$220k. This money then gets spend on mining costs, mostly electricity and mining hardware. The reward drops in half every 4 years so in 16 years it'll be 1/16 as much in bitcoin terms and if at that point bitcoin is around $500k the money spent on mining will be around the same as now.