In the PHB on page 270 we can read in the desription of Protection from Evil and Good that:
Components: V,S,M(holy water or powdered silver and iron, which the spell consumes)
Since the material components are consumed, they cannot be replaced with a spellcasting focus, as per the PHB (p.203)
If a spell states that a material component is consumed by the spell, the caster must provide this component for each casting of the spell.
This is reinforced by sage advice:
A spellcasting focus can be used in place of a material component only if that component has no cost noted in the spell’s description and if that component isn’t consumed.
The description does not list a cost in gp though. Which brings about the question above. While holy water is created in discrete 25gp "units", powdered silver and iron are not, so we cannot just assume that you need 1 unit of holy water (ie. 25gp worth).
So, is a sprinkle of holy water enough? (Allowing the caster to use 1 unit for multiple castings.)
If one would use silver and iron, how much would they need? 25gp worth? Just "some"?