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.