The marginal cost of each transaction in the block is approximately zero.
The only effort is from propagation speed, however thanks to the relay network that effect is almost nothing.
Say it is $0.15. A miner can make extra revenue at ~0 cost by filling a block with $0.10 offered fee transactions, but if there are many of them, they may choose not to, from the belief that enough of them will turn into $0.16 offered fees in the future (the cost is clearly ~0 in the short term, it's harder to say what it costs them in the long term to propagate the impression that low fee offers will still eventually clear).
Another miner will pickup those transactions and mine them.
Blocks are either full and there are fees or blocks are not full and fees are approximately zero.
Unfortunately the block size cannot be limited by individual miners.
(Which I realize is for a long time in terms of there being a block reward, but if bitcoin falls by 50%, so does the reward...)
(In general you cannot get 10+ MW of power without agreeing to pay for installed capacity and energy separately)