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.
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.