Payback period is the time for mining income to cover the cost of the device and running costs (mainly electricity). The formula is simple: (ASIC cost + optional delivery and setup) / (daily income - daily electricity cost).
Daily income depends on the miner’s hashrate, current difficulty and BTC price. Electricity cost depends on power draw in watts and the price per kWh. It’s important to assume a conservative case: difficulty usually rises, price can fall, tariffs can increase. Efficiency (J/TH or W/TH) shows how sensitive the device is to electricity price.
Before buying, calculate payback at different prices and tariffs and compare models by efficiency and price per TH.