For example, here is a reseller [0] that sells a 72V 120A (8640W peak, 4320W continuous) three phase pure sine wave motor controller for 126$, with warranties, logistics (plus shipping). I'm fairly certain that you could get a similar, or even lower price for only two phases and a fixed frequency at the same wattage. If you really want to use them as inverters though, they are programmable so you could set a fixed frequency, though I don't recommend it. I've heard of people running some other models at 120V, but I really don't recommend it :)
In any case, I think this shows that a 4320W inverter can be built and sold profitably for a lot less than 200$, if a 4320W+ three phase multi-frequency programmable inverter can be.
I should that 4320W is with passive cooling. With active cooling (from sticking a fan in the box to water cooling) you can get away with >12kW continuously.
[0]: https://www.aliexpress.com/item/32892079337.html?spm=a2g0o.p...
Presumably this is very inefficient, but a lot of power needs can be satisfied by running it for shorter periods of time to run compressors on refrigerators and charge batteries. I would imagine this should work very efficiently for a Tesla as long as the computer doesn't stop you.
In either case though you want to have a full tank/battery. If your Tesla is low it won't help much, so although it would work really well most of the time it would be difficult to rely on it. In a remote area it is usually relatively safe and relatively cheap to store some gallons of extra gas.
Perhaps what they are hinting at here about having cheaper recycled batteries (not in cars) will make more sense for these kinds of applications: cheaper powerwalls. But I agree it would be nice to be able to put power back into a power wall from a Tesla. Particularly given that the default setting will assume a temporary outage and start charging the Tesla from the powerwall.
[1] https://www.consumerreports.org/cro/2012/02/can-a-pint-sized...