What's the difference between the two?
'It costs $50 per person'
'It costs $50 each person'
It costs $50 needs an adverbial modifier if any is considered appropriate.
It usually costs $50.
It costs $50 Thursdays.
It costs $50 a bottle.
Adverbs, and the rare adverbial objective (noun used adverbially), fit the bill.
More commonly, to-infinitive clauses and especially prepositional phrases will be used:
It costs $50 to get there.
It costs $50 on the tram.
It costs $50 for a bottle.
It costs $50 per person.
It costs $50 for each person.
But
*/?It costs $50 each person.
would probably not usually be considered acceptable; it would be an adverbial objective usage, with the preposition 'for' dropped from the prepositional phrase, and this often does not give an acceptable-sounding variant. (This particular example may sound more acceptable in the States than in the UK. 'It costs $50 a person' would however be quite acceptable.)
'Each person' is commonly used as a subject. Eg: Each person must pay $50. But 'per person' is used as an 'object'. Eg: It costs $50 per person.
"It costs $50 each person" is not correct usage. "It costs $50 for each person (to go on the trip)" or rather "It will cost each person $50 (to go on the trip)".
PS: I am not sure if the terms 'subject', 'object' are correct in the context.