I bought a Lincoln MIG welder but not their cart, figuring I could make my own cheaper. Then I found out I could buy a passable one from Princess Auto for the price of the steel alone. IIRC it was a dollar a pound including all the hardware.Even if you don't overbuild it, a quick off-the-shelf parts list for a home-built trailer is already over $600 before tax (axle assembly, wheels+tires, leaf suspension, lights, coupler), and that doesn't even include the metal for the frame or the plywood for the deck.
A possible source of trailers is a trailer park. Sometimes people abandon wrecks and they get hauled into the bush. You end up doing a lot of dismantling but all the parts are there. They will probably need new tires and the axles may be too heavy duty for a light haul trailer. Camper trailers are better. Sometimes they appear on Kijiji or Craigslist, free or really cheap.