I'm building a 6.5x12 right now. Welded the crossmembers and leaf spring hangers today.
Not sure what trailer go for in your area but there's no way I am saving money. I'm just doing for the heck of it and like to do things myself.
I bought a single axle kit w/o brakes, but I did get the rims and tires - $420 shipped. Bought the metal, 2x2 and 2x4 sq. tube last week - $405. That doesn't include the expanded metal for the gate and will likely need more 2x2 for the gate frame. Also doesn't include the floor boards which I priced at Lowes today - $135 for treated, $90 not treated and about $10 for some Thompson Water Seal which I'll probally use.
When it's all added up I'm guessing around $1100 to build myself. I was qouted $975 for a 6x12 from a local trailer builder but that was using angle iron not tube.
You can do a google search for utility trailer parts and find what you'll need.
Have fun and good luck!
rb