Cairn has the right frame. Duration is hours, so the measure is closings per month rather than months outstanding. The same $200k that funds one closing Monday morning is back before Monday afternoon and available Tuesday. A funder doing four closings a month at $2,500 to $4,000 each on a single $200k line is running a completely different business from your eight-per-year sketch, and the gap between those two outcomes comes down to deal flow. Pricing barely matters there.
Which means the number you actually need is how many double closes your wholesalers put on the calendar per month, and how many of those you win against the other funders they call. The fee is secondary. That's a relationship question and it's lumpy. Most people who do this seriously carry several wholesaler sources because any one of them goes cold for a month at a time.
The part that breaks the recycling math is a closing that doesn't repay the same day. If the B-C leg falls apart after you've funded, your hours-long loan becomes a property you have a lien on and a note from a wholesaler with no balance sheet. Your capital is then locked for however long resolution takes in that state, which can be months, and every closing you would have funded in that window is gone. One of those wipes out a year of fees, so price the fee against the failure cost rather than against the six hours.
Also expect fee compression. The service is easy to enter and the flat fee is the only variable a wholesaler shops on.