This answer really depends on the current gas cost on Ethereum. There are ways to mine "cheaply" though. In the documentation, check out Proof Frequency and Mining Behavior sections.
The TLDR; is if you want to save on Eth cost, you mine with a lower Proof Frequency. This will indicate to the miner to attempt proofs with higher difficulty which results in higher yield per proof and with less ethereum transactions.
I know that there are too many variables to say for certain, but what would you say is the recommended ETH
The cost is 130,000 gas per proof which is around $2. The miner will also display below your ETH balance how long you will be able to mine given your proof frequency, ETH balance, and the current cost of gas.