There is a lot of difference between a car trailer and a dump trailer. A 7000# car trailer has a frame with decking. A dump trailer has a frame, no decking and then another entire frame that is the dump box. That's two frames. Plus the dump box has a continuous steel skin for the bed and higher framed sides with taigate (all with contiuous steel skins). That's a lot of metal and framing which means more cutting, welding and painting. Then of course there is the cost of the hydraulic system (pump, reservoir, cylinder, lines, wiring, controls and battery). The frame of the dump trailer must be heavier and with more structural supports than a car trailer because it must withstand the twisting and bending placed on the frame by the raised and loaded dump box.
My 6' x 12', 10,000 GVW dump trailer weighs 2800# empty. I doubt that you will find that even a 20' car trailer comes anywhere near that weight empty.
If you decide on a dump trailer, don't go cheap. Look at the frame and structural supports. If all you are going to load is light weight material such as mulch, not much of a concern. But if you want to haul dirt, stone or other heavy material you can easily overload the trailer's weight capacity before you fill the box. You want to make sure that the trailer has some "extra" capacity to lift, dump and support a heavier than rated load. Nothing worse than having the loaded trailer just sit there and groan.
Well that's my 2 cents worth.
Jeff