Your question complements another question: "
How much do I actually sell per month?"
With the answers to both of these questions, you'll know your
operating profit per month, which focuses on holding costs and not cost of goods.
Though,
Domain sales are cyclical. 500 domains isn't enough to think about this question in terms of
months. In all likelihood, you need to think about it in terms of
quarters or
years. Then you can calculate the arithmetic mean and ask:
How much do you sell
on average per month?
But I caution you against thinking in terms of months with a portfolio that size, averaged out.
Anyhow, these are fascinating questions that every business owner should answer.