If a delegator with let’s say 1M ADA decides to delegate to a pool with a current stake of say 100k, that delegator is going to lose quite a lot of ADA Return on his Stake. As I have shown in my post, What I learned after 1 month as a small stake pool operator, in such a case this delegator will get just about 65% of the rewards he/she would get when staking at a 50m (large) pool. This has mostly to do with the fact that the minimum (340 ADA) fees will weight down significantly on smaller pools, where with larger pools these costs are spread amongst many more delegator’s stake.
So to say it costs almost nothing is not really accurate. Or maybe you mean something else?