What is the difference between these two sentences?
I earn $500 each month
I earn $500 per month
The latter describes your salary: if you sit at your desk and don't get fired, you'll get $500. The former is more a description of the situation: somehow, by hook or by crook, I manage to scrape together $500.
The use of "per" imply a causal connection between the passage of time and the arrival of your pay. "Each" encompasses the possibility there's no connection, and it's just happenstance.
"Each month" is a fixed amount, "per month" can be an average.
The two are equivalent. Neither holds any implications as to how you come by the money; that is implied by 'earn'.
Per means 'for every' or 'for each', so it's almost exactly the same as just saying 'each'. Per is perhaps more common.
"$500 per month" connotes "I earn money at a rate of $500 per month." So if you work for half a month, you will earn $250. "I earn $500 each month" implies that from the start of the month to the end of the month you acquire $500.
I'd say that "I earn $500 per month" suggests that your monthly salary from a single source is $500, whereas "I earn $500 each month" could mean that you make $500 from potentially multiple sources.
There are subtle differences in meaning.
“I earn $500 each month” says that each month, considered separately, the sum of your receipts is $500. This can mean that the sum equals $500 exactly. But it can also mean $500 at least. For example, you can write:
I earn $500 each month. During peak months, I earn as much as $800.
"I earn $500 per month” express a rate, or linear relationship, between time and money. It means that over n months, the sum of your receipts is n × $500. Again, this can mean that the monthly sum equals $500 exactly. But it can also mean $500 on average. For example, you can write:
I earn $500 per month. This is considering the year as a whole. The summer months are slower, but I always make it up over the holidays.
There is no difference. The following sentences are equivalent:
- I earn $500 each month.
- I earn $500 per month.
- I earn $500 every month.
- I earn $500 a month.
- My [monthly] salary is $500.
From personal experience, I would say that per month is the least likely to be used in this context.