The main investment for the user is time and effort (which they have a lot of) raw materials are not that expensive and can be from salvaged pieces in some cases.
There were at one point people who made and sold the life-trac tractor as a business too.

From the development perspective it is the prototyping and documentation that is a grind, not all the designs are going to work the first time around.. so they sometimes have to go literally back to the drawing board...