The fixed vs. variable costs should be relatively easy to figure out, right? Let's say your ticket price is based on a fixed seat price + average pax weight calculation, including that of your baggage (yes, why not include carry-ons). When you buy your ticket, you estimate your weight and what you anticipate your baggage to weigh, and pay accordingly. Then, in the interest of true fairness, when you get to the gate, weigh in, and if you're over what you paid for, you pay the difference to board. If you're under, you get credited at the same pound-for-pound rate. If you're a 2-seater, and you only paid for one, pay up or stay back. Sadly, doing the math on this would be really easy, but no one has the guts to do it.
I agree with dashdog's point that weight has little to do with the actual cost, but if the ticket prices were broken out according to actual cost, and I saw that by weighing 180lb I only save $3 compared to the 350lb guy next to me on a regional jump, I won't spend the whole leg thinking about how I subsidized his use of the armrest and the extra biscoff he asked for....
As long as we're on this rant/idea, what if ticket prices had to disclose in dollar amounts where the money went? As long as restaurants have to tell us how much the food is going to kill us and where it came from, why not know that $2.50 of my ticket went to pilot wages, $86 to fuel, $12 to marketing, and $.40 to the guy who sucks out the lavs at the ramp? Maybe they should charge pound-for-pound for that :-)