If you are intending to add Stainless Steels to your repertoire, an electric HT oven is certainly the best choice.
However, if you are intending to stick with steels that can live with relatively simple single-temperature HT, a purpose-designed and home-built Propane HT forge setup might be a better option. "Forge" is a bit of a misnomer in this case, as it's not intended for actual forging.
By single-temperature, I mean that while the temperature is adjustable to that appropriate for the steel concerned, the amount of manual tweaking needed to set it to temperature means that Ramp/Soak HT regimes are not really practicable: It's a case of adjust to temperature, put in the workpiece, let it get up to temperature and soak for however long is needed, pull it out and quench.
I'm pretty lousy at taking notes and documenting stuff, but one of the test-drivers made a video of one I built a while back.
https://www.youtube.com/watch?v=1xvWkXBXY6U&t=9s
I have built 4 of these so far. The aim was to come up with something that would work well for the guys who want to be able to HT themselves, to a high standard, but who do not yet have the budget to buy an Evenheat or Paragon. It was mainly intended to work with O1, since O1 is easy to get in the UK and most other blade steels are not. The biggest difficulty most knifemakers face when HT-ing O1 is maintaining an accurate temperature for several minutes of soak time.
I had seen a couple of Don Fogg-style 55-gallon drum HT forges that worked well, but they were intended primarily for swords and were too big for most hobby knifemakers in the UK; I wanted to come up with something smaller.
2 of the guys who have had the HT forges have since progressed to electric HT ovens, at least partly funded by the proceeds from the knives they've treated in the gas forges.
It's not easy to estimate costs for building something similar on your side of the pond. To be honest, I find it hard enough to estimate the cost to normal folk over here of duplicating something I've built using mostly available (to me) materials.
The clever bit of this one is the burner. This is based on an "Amal Atmospheric Injector", a British-made commercial Venturi mixer, which cost me around 35 GBP (call it 45 bucks) from Burlen Fuel Systems as a walk-in customer.
http://amalcarb.co.uk/downloadfiles/amal/amal_gas_injectors.pdf
The design allows extremely fine control of the air intake relative to the gas flow. The net result is a Venturi burner that exhibits all the controllability of a very-well-executed blown burner. I have never seen a homebuilt Venturi burner that offers anything remotely resembling the controllability of the commercial units. Fine control of the air-to-fuel ratio allows temperature control that is close to that obtained with a PID-contolled oven.
There are a number of other makers of similar Venturi mixers around the world and I am sure there will be at least one in the USA, though the difficulty is likely to be finding a maker and getting the correct jetting.
For the HT forges, I found the 354/12 BLV, fitted with a number 60 jet (Amal sizing) and intended for use with Butane, gave the best results on Propane, though I think the one in the video may actually be running with a 30 jet. The 60 jet is a little easier to set temperatures with because small adjustments have a slightly smaller effect on temperature.
In addition to the atmospheric injector, the burner also comprises some pipe fittings (which came from my scrap box), the regulator, hose and fittings and a pressure gauge. All of this I already had.
The pyrometer cost between 3 and 4 GBP (4-5 bucks) delivered from China.
The thermocouple came from a local supplier we use at work and cost around 30 GBP (35-40 bucks). It's a Mineral-Insulated Type K, 6mm (1/4") diameter and 24" long below the handle with a type 310 stainless steel sheath. The sheath is good to about 1100 degC (2012 degF). It will take higher temperatures, maxing out at 2500 degF, though this will shorten its life greatly. Without the local source, I'd probably have opted for an Omega KHXL-14U-RSC-24. It is more expensive, but the sheath is claimed to be good to temperatures in the welding range.
The shell was a length of 10" thin-wall stainless pipe that I had in my scrap pile. I've also used a scrap compressor receiver and I'd expect pretty much any suitable length of steel tubing about 10" in diameter to be adequate.
The shell is lined with a single layer of 1" blanket, rigidized/coated with a homebrewed goop, and the ends are disks of 1" board, coated with the same goop.
Building with some of the stuff I've accumulated over the years, "finding" a shell and buying in the rest (atmospheric injector, regulator, hose, gauge, thermocouple and pyrometer), I have probably spent less than 100 GBP on each one. I am pretty sure I could build one for under 200 GBP, even if I had to source everything, and that "anyone" could build one for under 250 GBP. US prices seem to be generally lower than UK prices, so it may be cheaper your side of the pond.
