Well, if there's not a cost difference, why do they keep trying to double my pricing when the base tier goes up?

My sense is there is some cost savings at these lower tiers, but more importantly it would prevent ISPs from jacking prices up while using higher speeds as the rationale ("Look, we just tripled your speed, and it only costs double!"). ISPs would be less likely to double prices without any service improvement. But since many people don't care about the service improvement, it's an illusory benefit for them.