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Protection from Evil and Good has a material component of

holy water or powdered silver and iron, which the spell consumes

So, to make use of this spell we have to spend holy water (or powdered silver and iron, but I'll come back to this later), even though it doesn't explicitly state how much unlike many other spells.

For example, Glyph of Warding

incense and powdered diamond worth at least 200 gp, which the spell consumes

Or Snare

25 feet of rope, which the spell consumes

We do have a relevant tweet from Jeremy Crawford.

In practical terms, does that mean a flask of holy water could be reused for multiple castings of the spell? If a DM was generous and allowed that. Typically, a DM will expect a flask to be used.

And here we've reached our destination, from Holy Water (flask) description:

Cost: 25 gp

Now, given that powdered silver and iron doesn't have it's own position in the equipment's list and it's worth isn't explicitly stated in spell description either, it's price is completely up to DM, varying from 1 cp to infinity, which is ridiculous, to say the least, comparing to the fixed price of Holy Water. Assuming it should be at least of the same price as Holy Water, it leaves us with 25 gp for a one-time use material component for a 1st-level spell.

As it is, there are several 1st-level spells that require consumable material component:

• 
  Ceremony with a cost of 25gp


• 
  Find Familiar with a cost of 10gp


• 
  Snare with unspecified cost (it isn't clear what rope we can or should use)


• 
  Protection from Evil and Good with a cost of 25gp

Now, some might say, that Ceremony is expensive as well, but it also provides wide range of options both for combat and roleplay, while Protection from Evil and Good is hardly applicable to anything other than combat encounters.

Am I missing something or does this spell really cost 25 gp per casting?

First off, I somewhat disagree with Jeremy Crawford's tweet. Since no cost or amount of holy water is listed, I (as DM) would not expect you have to expend an entire flask -- I would expect it to be something like what you'd see in a modern Catholic church, where you only use enough holy water to wet the fingertips (or a pinch of iron and silver); a minuscule amount costing effectively nothing per casting as long as you have an appropriate component on hand. Dumping an entire pint of holy water on somebody seems excessive, to my mind. You don't anoint people with a bucket.

But, supposing your DM does decide this spell costs 25 gp of holy water or an equivalent amount of silver & iron per casting, I still don't think it's "too costly to use". It might be tough at first level, yes, but if you're getting gold income at a reasonable rate, you should have plenty of liquid cash by 2nd level to cast it quite a few times, and by 5th level a 25 gp cost is, if not completely irrelevant, at least low enough to be entirely reasonable for such a strong effect -- it's somewhat situational, but within the appropriate situation, Protection from Evil and Good is hands-down the best defense spell at 1st level.

The explanation of the item Holy Water says:

A cleric or paladin may create holy water by performing a special ritual. The ritual takes 1 hour to perform, uses 25 gp worth of powdered silver, and requires the caster to expend a 1st-level spell slot. (PHB page 152)

So Holy Water is worth 25 gp worth of powdered silver. But there is another rule which tells you that you don't even have to worry about your components if casting Protection from Evil and Good.

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. (PHB page 203)

With that rule you have to provide either Holy Water or 25 gp worth of powdered silver plus iron. Just be realistic with the iron: No cleric or paladin is carrying around tons of iron.

About material components

A character can use a component pouch or a spellcasting focus in place of
the components specified for a spell. But if a cost is indicated for a component, a character must have that specific component before he or she can cast the spell. (PHB page 203)

If you have a component pouch or a spellcasting focus you only have to worry about components with indicated cost or which are consumed by the spell. As Protection from Evil and Good doesn't indicate any cost but consumes it's components you have to worry about them.

Note: If you have no component pouch or spellcasting focus (maybe because it was stolen) you always have to worry about components, no matter if any cost indicated.