"Serving" is (derivationally, semantically) ambiguous in English: it can be a nominalization ("serving children beer is a bad idea") or it can be a noun that is only historically related to the verb "serve". "Per" is actually useful, since in the reading "for each", it requires a noun referring to a measure, not an action. The lexicalized measure noun doesn't take an NP complements but the nominalization does. (I assume you can take it from here)