j15t

Each operation in the Ethereum Virtual Machine (EVM) has a cost defined by the protocol. Miners verify that each transaction has paid the necessary amount and any transaction that pays too little will not be included in a block.

> In particular, I'd like to understand whether mining fees are paid for executing contracts or for solving hashes or for both

I believe the answer is both. Miners get a rewarded for finding a valid block of 5 ETH (typical PoW style reward) and they also receive the transactions fees paid by users of the network. And in Ethereum the cost of a transaction is determined by its 'gas' cost, which is based on how computationally expensive the transaction is.