A bike + trailer is the most economical. A purpose built cargo bike will have a lot of advantages over a bike + trailer, cost is definitely not one of them. They come set up with gearing and brakes to haul heavy loads, kids are in the front of the driver, they are large and cars pass them cautiously, you can hook a trailer up and carry 4 kids and groceries... although e-assist or no hills would probably be a prereq for this setup. At the moment they hold value really well, and lots of the cost can be re-couped when you no longer need it, however cost will come down as they become more popular, and don't have to be imported a bike at a time.
Mostly I love cargo bikes, because they help make biking instead of driving for transportation and errands more main stream. There is less to figure out, and it's more easily adopted by biking neophyte.
I am however a biased owner, we picked up an xtracycle after a year of bike commuting. We are on track to have driven our second car less then 300km since October... I used to drive about 600/month. We are not quite ready to call it quits and sell it yet, but I can see that there may come a time that we can.