You can definitely find this for a bit < $200, its just kind of hard to find a trustworthy brand (maybe you know of one)? I think these may be reasonable markups to actually run a business with a finished safe product, to pay employees, ship these things, and offer warranties and handle returns.

Kelly controller is a pretty reputable brand.

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...