It would certainly be cheaper to build something specifically for your needs, even if you use a commercial Venturi burner, and it will almost certainly use less gas.
90,000 BTU/hr is very close to 91,500 BTU/hr, the heat obtained from burning a gallon of Propane/hr (4.23 lb/hr). This seems to tally with Ed's "fuel hog" description.
It looks to have a blown burner. There are usually two adjustments that can be made to regulate temperature: The amount of gas being supplied (adjusted at the regulator or with a needle valve) and the amount of air that mixes with the gas. It's not clear from the photo whether or not there is any adjustment on the regulator or just downstream of it. The air adjustment is by moving the choke plate over the blower inlet.
The flame temperature is actually a function of the fuel:air ratio and can be adjusted very finely on a well-designed/built burner.
I built a dedicated HT forge for a guy who was having difficulty holding a soak temperature for treating O1 steel with his previous setup.
He posted a brief youtube video that should give an idea of the temperature control attainable even with a somewhat quick-and-dirty setup. The temperature display is in degC. Target temperature was 816 degC (1500.8 degF).
https://www.youtube.com/watch?v=1xvWkXBXY6U&feature=youtu.be
Propane usage at the settings used in the video (20 PSI with a .020 gas jet) is around 1.2 lb/hr
The Venturi mixer is a commercial one, branded Amal, and cost around 35 GBP (55 USD) as a walk-in customer. I've stopped trying to build my own Venturis since I found out the commercial ones are so cheap.
