Cost isnt a "Prevention" unless you
cannot pay. If I have 1400 life points, or only 2 Continuous Spell Cards on the field, I am prevented from Summoning Hamon, in whatever scenario you agree with concerning
Chain Energy.
An "effect" or "condition" prevention is different as it can only be ignored if it is no longer active (or "true"), or does not affect the player due to a conflicting or inhibiting effect.
You cannot Normal Summon or Set
Rare Metal Dragon. I am prevented from this action, but I am not prevented from summoning him altogether. I can still Special Summon him.
Chain Energy only prevents a summon under one instance: If you cannot pay 500 per active
Chain Energy. It does not tell you that you cannot summon a monster.
Rivalry of Warlords prevents the summon of a monster that is not of the same type as that on your side of the field unless it replaces it, so its effect is only preventing a summon if you have a different type monster in hand and cannot tribute it, otherwise, there is no prevention. So, Rivalry says you cannot summon a different type monster, while
Chain Energy says you simply must pay to summon.
I dont think the two are comparative. In one case, I cant summon a monster to remove
Rivalry of Warlords because it would mean that I am summoning a different type monster while another one already exist, and the other hand is just saying that in order to play a monster from hand, I must pay as long as
Chain Energy is active.