I decided to settle this once and for all. On 2/20 @ 15:00 EST or so, I went to Spirit (NK), Delta (DL), and jetBlue (B6) websites and looked up every day in March 2013, a OW FLL-LGA ticket. While hardly a representative sample of the entire fare structure of each carrier, it settles this argument, at least.

The envelope, please:

Just the prices presented (all government fees and fare) show us:

Cheapest average price (mean):

1. NK = 137.90
2. DL = 161.10
3. B6 = 170.55

50% of the fares are above this, 50% are below (the middle fare, the median):

1. NK = 94
2. DL = 149
3. B6 = 169

You're most likely to get this fare (the mode, the most often occurring number):

1. NK = 94
2. DL = 169
3. B6 = 203

So, just looking at the fares that a passenger would see, Spirit is definitely the cheapest, then Delta, then jetBlue. I wouldn't call it 1/2 as cheap, but certainly noticeable.

However, lets add in reality. Lets assume this passenger has 1 checked bag, 1 'personal item' that fits under the seat, and wants 1 beverage on the flight. Spirit is going to tag you with a $30 fee for the checked bag, a $3 charge for the drink, and is also going to get you for $15 in fees ($13 "usage fee" and a $2 "DOT Unintended Consequences" fee). Delta will charge you $25 for a checked bag, but you get the drink free. JetBlue will give you the bag, and drink, for free. So, with the new, more realistic #'s:

Mean:

1. B6 = $170.55
2. NK = $185.90
3. DL = $186.10

Median:

1. NK = $142
2. B6 = $169
3. DL = $174

Mode:

1. NK = $142
2. DL = $194
3. B6 = $203

So, at the end of it, Spirit is still the cheapest. If you could manage to travel without a checked or carry-on bag that had to go in the overhead bin, and chose not to drink anything, Spirit would only enhance their position as cheapest. Spirit has the lowest cost on any given day (mode), the lowest mid-point cost (median), though, if you travel on them on the 4 days of March where they are the most expensive, you will pay for it (hence the #2 ranking in mean). Both Delta and JetBlue are more expensive, but both could make the argument that they are cheaper than the other. I'd give the tie to jetBlue, only because jetBlue has the #2 median (the most telling # from a mathematics point), and jetBlue does have the lowest average cost, where Delta doesn't lead any category. And, of course, both Delta and jetBlue offer what is, unarguably, a superior product (more leg room on both, TV's on jetBlue, milage program on Delta, etc.). Is it worth the extra $?

2 random notes.

Spirit has the most deceptive pricing, IMO. Their 'unintended consequences' fee is b/c the DOT requires all airlines to refund a ticket purchase for 24 hours. While they separate it out (which does alert the consumer to the consequences of the DOT's action), every other airline puts this in the fare #. The 'Usage' fee is just going to Spirit's bottom line, so should really just be part of the fare. Delta publishes a fare that is always $XYZ.80. But, when you just look at the whole #'s, Delta, instead of rounding a $149.80 fare to $150, rounds it to $149. This places the fare higher on the list in a 'from lowest to highest' fare comparison website, ahead of say, jetBlue. On 14 days of the month (almost 1/2 the month), Delta's fare is $.20 cheaper than jetBlue's fare, but, due to this 'rounding', Delta's fare appears first on the comparison between Delta and JetBlue all but 5 days of March.

All of this comparison is only the lowest fare offered that day. That's often a very early or very late flight. If you want to fly during banker's hours, then that will usually cost more, and may change the rankings.

Incidentally, on the one day in question, 3/11, I found Delta @ $214, jetBlue @ $203, and Spirit @ $139 - this is all before the non-governement fees.

I'd be happy to send the data set to anyone who wants it (PM me) - though, honestly, does it surprise anyone that Spirit is cheaper, and both Delta and jetBlue essentially match each other's fares 50% of the time?