I have been using salt pots for seven years. I would not consider doing my heat treating any other way. I also do commercial heat treating for other knifemakers using them, and have heat teated a bunch of not-knife stuff for folks around here who had things that needed to be done.
Essentially, the high temperature rig is a gas forge standing on end, with a temp controller that operates the blower and a solenoid valve for the gas. Or, alternatively, if you are not going to do high volume, a pottery kiln or other electric resistance heating device will do the job nicely. Cost to run depends entirely on what you use. My gas high temp runs for pennys an hour, but I would not care if it cost dollars an hour, I would still use it over other methods for low alloy steels, it is the best way.
I also quench and temper in low temperature salt (with steels/section sizes that will harden this way, not all will work well for this method, specifically 10xx steels are at the margin for hardenability, and I usually quench them in polymer, them temper in the low temp salt.) My low temp rig is a horizontal pan, with a pipe burner under it (see Brownell's hot blue set-up for an idea of what this looks like) and it is controlled by a Robertshaw commercial oven thermostat that is a throttling type thermostat. That means it throttles down the gas flow to a minimum flame when it reaches set-point, rather than shutting completely off, this eliminates the need for a pilot (unless you are real safety conscious, but in that case you should probably not build any salt pots any way, they can hurt you badly, more about that in a bit.)
Advantages include, NO SCALE not ever, you can finish the piece as far as you want to. I take damascus folders to a minimum of a 600 grit finish, and sometimes 1000 before heat treating. Seldom do things warp, this is especially true for those pieces that get quenched in the low temp salt at 450F. If they do warp, you can straighten them while still hot at 450 after the quench, they are not hard until the temperature drops down to ambient. At the time the blades come out of the 450F quench salt, they are in a condition describes as "metastable austenite". What that means is "not hard, but going to be real soon" in practical terms. The thoery s that you cooled it fast enough to avoid the formation of pearlite, but did not continue the cooling low enough to start forming any martensite yet, or very little. Ms (martensite start) temperature is different for every alloy compostion, and changes with austenitizing temperature a bit, but for MOST low alloy steels, it is right around 400-475F, most of the time.
Temperature control from this method is unparalleled, and allows one to manipulate the microstructure a great deal more effectively than "eyeball" methods. It is especially good to use if you intend pursuing 52100 as a blade material. What that steel does in heat treatment is very dependent on austenitizing temperture, and the results can be dramatically different with as little as a 100F change in the temp prior to the quench. If too much of the available 1%C is dissolved into the austenite solution, you will get "plate" martensite, which is more brittle, always, irrespective of actual tempered hardness as measured on the rockwell scale. Austenitizing temperature for 52100 should normally be held to 1550F or less. At 1550F, there will be .55-.6% of the available carbon dissolved into the austenite, with the remainder present as retained carbides in the martensite that is formed in the quench. This is good. Over-heat the blade to 1650-1700F (real easy to do by the "eyeball" method, and you may put as much as .65-.8% of the carbon into solution. This is bad. Plate martensite results, rather than the more desireable "lath" martensite, the retained carbides are fewer, or not there at all (which reduces the materials wear resistance or edge holding ability, and IMO reduces the potential "sharpness" of the blade as well.
Temperature controls are not quite as important with other, simpler, lower carbon steels as with the 52100 I chose as an example, but using salt allows you the greatest degree of precision in temperature control available to a low-buck operation, keeps the work scale-free, reduces or eliminates warpage, has incredibly fast heat transfer rates, allowing very rapid austenitizing. Using 3.5" diameter tube full of salt, operating at 1500F, a large bowie knife can go from cold to 1500F in under four minutes, with absolutely even temps throughout the entire blade. Small blades, like a folder blade 1/8" X 3/4" X 4 will come up to heat in under one minute.