I have twins, so adjust accordingly, but it looks like we're averaging ~$150/month on formula, diapers, wipes, soaps, lotions, baby detergent, etc., which are the items I think I'd put in the "on-going operation and maintenance cost" basket.
That said, we spend more than that per month on baby-related costs. I personally don't find it very helpful to strip out the one-time purchases because even if you only buy a certain item once, there will be another "one-time purchase" the next month and the month after that. For instance, we got a stroller in July because the babies are now big enough to ride in a stroller outside their car seats. We won't buy that particular thing again, but soon they'll be crawling and we'll have to get baby gates. We won't buy that particular thing again, but soon after that they'll be walking and we'll have to baby-proof. We won't buy that particular thing again, but then they'll outgrow their infant car seats and we'll need to get convertible ones. Technically these are "one-time purchases," but they recur pretty regularly, even if the item you're buying that month is something you've never bought before and will never buy again.