With a little more data the monthly average profit came up to 0.75 billion and stuck there, so unless it changes much in the next week I'm going to call that the correct number and stop tracking it.
The question now is how much time I have to spend per month to keep it going. It might take a few months to get a feel for this.
Also, I updated my stocking tool to show how many of each item it's set to want in addition to how many I'm short by. The goal of this is to avoid restocking items which are nowhere near being sold out, which will substantially reduce the amount of time I spend buying items and recreating orders.