Cost: Nada. Difficulty: only finding the parts and figuring out how to attach them.
This owes much to the Jackhammer headphones entry. I made a pair of those, complete with acorn nuts bolted to the plastic of the earmuffs to make them look more industrial. Did not do the trick.
I then thought of making headphones from scratch, or at least making earmuffs from scratch and putting in the cheap speakers later. Here is the result of that venture...
Step 1: the stuff I used.
I'm not going to take them apart and document every piece, but I'll go into the stuff I made them out of.
For the headband, I used, of all things, a steel water hose that would normally be attached to a toilet (it was new, not used), and snaked a steel rod through the braided hose, to be bent to fit my head later.