The above does unfortunately not talk about cost to the delegator incurred by pool rewards.
Lets assume we only consider pools with a 0% margin. The cost to the delegator still varies significantly depending on pool size …
340 / (64 x 750) => 0,7%
340 / (32 x 750) => 1,4%
340 / (16 x 750) => 2,8%
340 / (8 x 750) => 5,6%
340 / (4 x 750) => 10,12%
340 / (2 x 750) => 20,24%
340 / (1 x 750) => 40,48%
Assuming a pool has a 0% margin and that the block reward is 750 ADA, it follows that for every epoch a pool mints at least 1 block the owner will keep 340, which will not be distributed to the delegators. i.e. delegators will loose a significant part of their rewards by delegating to a small pool instead of a saturated pool that makes many blocks per epoch (on average). The minimum cost for all pools is a constant of 340 ADA, which is for larger pools a much smaller slice of the total reward that is taken away before the rest is distributed.
(Strictly speaking, the above is not quite correct for the pools at the bottom, because according to the Bernoulli distribution the likelihood of winning zero blocks in any given epoch is still quite high for a 1m pool. Other epochs will win more that 1 block, which brings the average anual cost down)
Lets also talk about pool margin … If a pool reliably mints at least one block per epoch, the owner receives 6 x 340 => 2040 ADA per month for running two simple nodes. This is far more than anyone should need to run a pool. One could argue, that this amount of pool reward is already plenty and it is therefore hard to understand why a margin of > 0% is needed on top of that.
Still, there often is good reason to charge > 0% margin for example to use some/all of that to do some good in the world. When deciding which pool to delegate to, this can be a decisive factor in line with - “Making the world work better for all”.
Recently, we have also seen operators thinking about ways to reward delegators with extra payouts from their pool rewards. I’d say, this is only natural when the cost of running a reliable pool stays more or less the same in dollar, but the value of fixed pool reward multiplies.
While running a pool on reliable hardware+infra is important, it is perhaps also important to have a look at the pool’s mission and whether/how the pool operator distributes that excess of pool rewards.