Seems that almost every city pair I picked it said that Southwest was the "only" low cost carrier serving this route. So, I picked STL (where I'm based) and BHM (where my most of my family live).
Here's what I found by comparing Orbitz fares and SWA (since their fares are not available on other travel sites).
SWA does in fact have the only non-stop service. One flight per day. And, they show an advance purchase fare of $208 round-trip. However, this fare does not include departure taxes, security fees, etc. There was one of the fees that their website says could be up to $18/ticket.
On Orbitz, CO and DL, show a "drive-away" fare of $215 for the same travel dates (NWA-$218, and US Airways-$224). This would include all taxes and fees. However, all involved at least one stop at IAH, MEM, CVG, ATL, or CLT. All (except CO) required a change of planes, and all involved at least one leg on a regional airline--CoEX, DL Connection, US Airways Express, etc.
FYI, per Orbitz, AA for same dates was $538 with change of planes at DFW, but it was all on "real" airplanes.
So, I guess ya picks ya poison.
P.S. Don't really know how accurate the passenger load info is. (Which I believe was your actual question.

)