I'm going to build my micro trailer and would like what you used, and cost.
Here's the link to what I did for my little trailer:
I tried to write it as a mini-trailer primer. Read thoroughly the above thread to get an idea of options you can take and their costs; between that link and this current trailer build write-up by Bobdog they should answer most of your questions. Especially check out post #27 on the provided link. With the Black trailer you can get just the bare frame with axle and tires & rims, and then add metal floring (or plywood), and metal stake sides (or make your own out of lumber or whatever). The red ones don't have optional metal flooring or stake sides, but their buy-in is cheaper. The Black trailers will start out about $100 or so more than the Red ones (Red ones most often purchased from Harbor Freight).
These little trailers are like Jeeps in that you can take a base model and customize it to suit your needs and budget. I went with the "Black" trailer; this current thread started by Bobdog is based on a "Red" trailer. He can weld (and I can't), so he can do things with his trailer that I couldn't do with mine. That's part of the fun.
I've got a little over $1000 in mine, but I went overboard to show what could be done with a mini trailer as a starting point; plus I needed a little trailer and wanted a project. If you subtracted the wider axle and larger tires and rims you could shave another $400-$500.
When I did my little black trailer initially, it was $250 about and that included the metal floor. With the added wooden stake sides and spare tire, and tag I had about $450 in it. Putting the larger wheels & tires, and wider axle, spare tire carrier, etc. it put it over $1000.
Hopefully with a little reading it will answer your questions. Of course please ask us if you still have some questions (now or after you start your trailer project). And if you do a little trailer project, post a picture with a brief write-up so we can all see what you did. It might give others an idea or inspiration.