Incremental cost though, on an extra 500lbs (say a 250lb SE100k with 210lbs checked bags + 40lb between carry on, coat, personal item) is much higher than $10.

A pilot friend said on a long fight, arriving with an extra ton of fuel might require 1.5 tons of fuel at departure to account for the extra weight.

I had seen some charts and calculated this before--IIRC, a 737 used approx 40 gallons per incremental passenger+bags on a 5 hour flight.