The earnings from a Lease are not fixed. Forging in Waves is modelled after the Nxt Forging Algorithm. It works more or less like this:

The probability for the generator to create a block depends on the stake of the pool. Probability = Total generating stake / Pool generating stake. The higher the stake the higher the probability for generate blocks

The fees for each transaction contained the mined block are transferred to the generator of that block. The fees in each block vary. Some fees might be paid in assets, some fees are higher (DEX operations and token creation)

In the past, Waves Platform project wanted to reward nodes without increasing WAVES supply with new blocks of coins. So Miners Reward Token (MRT) were distributed to miners every block. This is not the case anymore. Right now a lot of airdrops are meant to incentivize people to hold Waves.