The average American drives ~14,000 miles per year. People who get electric cargo bikes tend to be able to substitute the bike for about 20% of those, or 2800 miles. Counting gas and vehicle depreciation you can expect to spend $0.65/mile to drive, so choosing the bike avoids $1820 in expenses. You'll probably need to spend $100 or so on things like electricity, replacement tires, chain lubricants, and maybe a shifter cable or something in that time, so you end up with about $300 in your pocket you didn't have, along with a big smile.

You'll then end up with around $1800/year for the useful life of the bike.

This is an insanely good deal.

That's unfortunate, but I'm not sure it's reasonable to hope for much less -- I was also considering DIY, but based on what I saw shopping around online, even just the electric parts (motor/battery/controller) alone, sourced straight from China, are more than half of that cost. And then if you subtract that out, the rest of it would be a complete non-electric heavy-duty cargo bike for <$700, which itself would be damn cheap. It's hard to see how that cost could be cut much more, either: there's a limit to how cheap the materials and manufacturing costs can get, after all, as well as a limit to how much you can reduce component quality without compromising the safety of the thing (e.g. ditching the hydraulic disc brakes in favor of rim brakes on a bike designed to carry 450 lbs might not be a smart idea).

Point is, it might be expensive relative to what you can afford, but IMO it certainly isn't expensive compared to the raw costs of materials and labor that go into manufacturing it.

Anyway, if you live in a part of the world that's so poor that new bikes in general are out of the question, then yeah, I guess you'll have to wait a while for cargo e-bikes to become more available in the used market.