Depends on the PRC, but most full progression PRCS have this text:
When a new XXXXXXX level is gained, the character gains new spells per day (and spells known, if applicable) as if he had also gained a level in whatever divine/arcane spellcasting class in which he could cast spells before he added the prestige class level. He does not, however, gain any other benefit a character of that class would have gained. If a character had more than one arcane spellcasting class in which he could cast spells before he became an XXXXXXX he must decide to which class he adds each level of XXXXXXX for the purpose of determining spells per day.
- So I would say that PRC "caster" levels count as paladin levels(1/2 each) for determining your spells per day/spells known.