Increasing costs of 1G, 5G, 10G, <maybe something in-between>, and then 50G ever after.
Do you want your paladin to be a tank, a damage dealer, or an armored caster? Retribution is for adding damage, protection is obviously for tanking, and holy is for being better at casting your heals and having more mana for blessings/seals.
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Only the exceptions can be exceptional.