4'x8' Harbor Freight utility trailer runs under $600 for the 1700# capacity, under $500 for the 1200# capacity. Add a sheet of plywood and you have a new cheap flatbed utility trailer. Add in a couple hundred for PT lumber and brackets, you can now have removable sides.
I have one that I actually made 5' wide. The prices were much less when I did mine in 2016 but I did mine for under $500. I could probably be talked into selling it now that I have the car hauler trailer and a Gladiator.