I built a combi trailer for an Enterprise about ten years ago, just before I changed classes to the Wayfarer.
I studied lots of pictures of trailers,collecting measurements and formulating a plan of attack. After buying all the hardware(trolley wheels etc.) I turned the boat upside down and built the trolley using the bottom of boat as guide. I already had the wheels,suspension and hitch from a box trailer that I had stripped, and using the trolley as a guide I welded up the trailer. I think the galvanising cost me just under £60.
You can certainly make a combi trailer for less than £750.