[oil pan 4]

If you are the company that builds trailers in bulk to sell at lowes, home depot or northern tool you are trying to get the cheapest and there for thinnest steel that should do the job and hold it all together with as few welds as possible, as if there is a shortage of welding sticks.
Since the steel its self and welding are among your top expenses that is where you cut corners.

If you get your steel comes from the scrap yard you error on the side of heavier steel, part of the reason is the flimsy stuff like what the big box stores use in their trailers gets crushed or bent beyond the point to where its not useable ether because it failed in its past life or in the scrap yard environment its self. At 15 cents a pound you don't really worry about if you go 20 or 30 pounds heavy.
I think my standard trailer frame design will measure 98''x64'' that way it can easily swallow 4'x8' sheets of anything that's important because most builders only think in 4' and/or 8' lenghts. The box and A-frame alone will weigh 140 pounds and cost me a grand total of about $20 in steel.
The other expense is welding. Since I am the welder and I have pride and give a damn about my work I fully weld most joints for a few reasons. Also after years of MIG welding experimentation I have worked out the most economical way to lay down a weld that is both strong and good looking.
I'm thinking each one will run about $3 or $4 worth of wire, another $3 or $4 worth of my special economical welding gas mix, to stick together $20 worth of steel, $20 for trailer lights, about $100 for new springs axle, tires and spindles.
Not sure how much sandblast and painting will cost but I think it will be around $30.
Axles, spindles and springs are going to a bit of a money eater.
All costs are passed down to the end consumer, would like to pass along less cost.

There is literally tons of slightly used fairly straight hot rolled 1/4 inch thick C-channel, in the form of 5'4'' long scaffolding at the scrap yard down the street from me.
Plus there is all the other random stuff that shows up. I like the I-beams.
I think I will make the non standard trailers out of a mix of 1/4'' scaffolding and what ever shows up.

I think any one that owns or has access to a scrap pile, welding machine, drill and chop saw can build a trailer.
I find a lot of people lack electrical skills or are afraid of wiring, if so pay some honky mechanic $50 to wire it up for you.