Ive been wanting to get a cargo bike to do the school run for years but have been put off by the prices. Cheapest brand name cargo bike you see is the Yuba Kombi - which is $1800. It is unassisted, so you supply the pedal power. Basically a normal step through frame bike with basic components.
Tandems are way cheaper. You can buy one of those for closer to the $1000 mark. Why should cargo bikes be more expensive than a tandem is anyone's guess. Thats just for the bike, by the way. Just wait till you see the accessories. Feel like paying $100 for a small sheet of plywood? Well have I got a proposition for you.
I hasten to say that this is the most affordable option. All the others, Yuba, Surly, etc, are easily double the price - hardly any of them appear on the second hand market and when they do they are snapped up quickly.