I made my trailer for under 20 bucks. I had to buy a couple cans of paint, a sheet of sandpaper, a spring for the hitch, and a replacement bolt & knob for the hitch. I already had some wire panel. Those are about $15 each, so you should be able to build this trailer for under $100 easily, even if you need a replacement hitch or something. I'm going to add a reflective sign or panel on the back for going can collecting and fishing.
I suggest a steel frame trailer with 16 or 20" wheels, quick-detach wheels, quick-clamp hitch, and folding capability. The Safety First model is easy to find for less than $50 even in good condition. I got one from the trash for free!
UPDATE: This trailer is still in use, and working well. The rubber/plastidip on the clamp has come off, so I carefully padded the clamp with Gorilla Tape. This held up all day in 100+F heat, and did not scratch the bike frame. Make sure the corners of the clamp are protected or they will 'bite" the frame.