Both a and per are commonly used with meanings of "each" and "for every." Thus one may write "once a minute" or "once per minute." Because per comes from Latin, old-time grammarians insisted that it should not appear before nouns in English, but this restriction no longer applies. The appearance of per is widespread and acceptable in all commercial, economic, and statistical writing. Using a (or an) for per in a sentence such as "The yield per acre per year is 200 bushels" is not incorrect but does sound repetitious and overly refined. In most instances, a is an acceptable substitute for overused per and is preferable in such expressions as "once a week," "50 cents a gallon," and "Admission is $1 a person."
Dictionary of problem words and expressions. Harry Shaw. 1975.