This is the lifetime profit of all orders for this listing minus the lifetime costs of this listing. We currently compute two costs: marketplace fees and PriceYak repricing fees (remember: PY order fees, final value fees, and PayPal fees are already considered in the individual profits for each order).
If an order has multiple items, its profit is broken down according to the proportion of the total destination revenue this item represents. e.g. If one item is $50 with quantity of 2 and one item is $25 with quantity of 1, and the order had profit of $10, the profit for the first item would be 100/125 * 10, or $8.
In all cost calculations, number of months listed is a continuously increasing variable. That is, it goes up by 12/365.25 per day, not one each month after the start date.
If a listing is inactive, costs will be 0, as we don't actually have an end date available. This should be fixed in time!
Marketplace fees are computed by a very simple multiplication: $0.05/month * number of months listed.
PriceYak fees are computed by multiplying the current per-listing fee for repricing (if repricing is currently enabled) by the number of months that the listing has been listed. This could be improved by taking into account the previous fees and repricing status from billing_snapshot or listing_count_snapshot, but this is alright for now.