Go on Pro Line Trailer sales web site they give prices Can give you an idea of what the costs are..... Usually the dealers around here do not deal much if any on price for new enclosed trailers. 6 x 12 about $2000, 7 x 16 about $3000. They have about 100 trailers on there lot & more than one lot in different states. We have probably 5-10 trailer dealers within a 50 mile radius of here (Roanoke Virginia area) & at least one trailer factory / manufacturer
Keep in mind all trailers are NOT made the same, some are poor quality some are ok & some are built very well. I have a 1980 5x8 enclosed trailer built by FRUEHAUF with the same quality that their tractor trailers are built for a rental fleet. I have bought most of my utility trailers used. Usually from some one that doesn't need one any longer. Looked at a homemade enclosed trailer that had fence posts as part of the frame (I did not buy it).
I missed out on two used car haulers that sold as repossessions at $1000 each.
As far as insurance the tow vehicle covers the liability when connected to the trailer but the trailer can have its own insurance for theft/ collision / comp. but usually not required.
Good Luck !!!!