Here's my contribution to the Homemade Cargo Bike revolution!! :) Used an old Mountain Bike frame from a great junk bike store close by. I basically followed the instructions from this guy: https://www.instructables.com/id/build-a-2-wheel-cargo-bike/
with a few of my own modifications. The bike is functional, but not done at this point. I still have to engineer and build a kickstand, install the brakes and shifters etc, and decide what kind of platform or box I would like to have.
Note, I brazed the entire frame, which I have never done before, but since I didn't have a working TIG welder I went with this because of better control than with MIG or standard oxy-acet welding. The joints are not all pretty, but they are strong and beefy!!
Materials used: Mountain bike frame, 2 BMX frames (used for the steerer tube and the 20" fork). Cost: $20
Some scrap steel tubing, I used various cutoffs from the local steel yard. Cost approx. $30
Bike components: had a bunch lying around from my various bike projects over the years, bought some others. Cost: depends on how high end you want to go.
Next I used some thin walled 1 3/4" diameter steel tube for the main frame extension off the front. Figured this would be nice and stiff since I'd seen lots of variations where people tried to engineer multiple tubes together to add stiffness. This worked great. This also allowed enough clearance so I could keep the entire length made from 1 piece of steel instead of 2 separate pieces.
Harbor Freight Tube Notcher was a great investment (with some slight modifications to make it more accurate: http://www.youtube.com/watch?v=Ty915Z0GrJo