The US carriers are supposed to be notorious for really good management of flight prices, especially United (although maybe not so much on their domestic flights where they just kind of dump seats into the market to get the competitors out) but I remember reading somewhere about a computer algorithm-y thingy at UA that looks at each individual flight and weighs up every last cost so that they can charge down to the cent the correct price to turn a profit...
With that in mind I'd be confident in saying anywhere from 50-60% LF's will be turning a profit for them