When your mining power goes to the NiceHash pool, you earn BTC for every share your virtual miners deliver, at the index price in BTC/EH/day.
E = P × H × t
where E is what you earn in BTC, P the index price in BTC/EH/day, H the hashrate your shares deliver in EH/s, and t the time in days. For example, the Rig 30d package mines at about 1,275.72 TH/s for 30 days. At an index price of 0.4763 BTC/EH/day it earns:
E = 0.4763 × 0.00127572 × 30 = 0.01823 BTC
This is the estimated earnings shown in the package details before you buy.