The cheapest way is not the fastest way, and the cheapest way also takes the greatest skill, and the cheapest way takes the same amount of preparation.
I’ll assume here you want to build a steel frame as most aluminum assemblies take some very expensive post-treatment to anneal and retemper, and that isn’t cheap at all.
Do you STILL want to know the cheapest way? OK. It is brass brazing, with lugs. This process requires a good 3 or 4 hours of joining for a very well skilled person using a proper oxy-acetylene torch. Good builders have a specific procedure, most of which also require each joint to cool before goint to the next one place rocker panels without welding. This is an example of a lugged frame.
