Comercial galvanizers I know have a $200.00 minimum. The price went up a lot in the last three years. (The EPA charges you for the right to polute). The price is generally figured by the pound. Almost as much as the original steel, so say 25 cents per pound, and a surcharge if it is tubing...so a 300 pound trailer of channel section is $75.00.
The good thing about zinc is that where there is none, the rest gives it an anodic protection?
On trailer design: I'd say that a trailer needs to be as small as possible. It is too long to be "stiff", because even without the boat, it flexes. Clearly the payload is not the issue with flexture. A drawbar that attached to the front crossbar (where the weight acts)with a minimal support to the back crossbar and mast cradle, with a rubax type independent axle would be the way I'd go. Some sort of rotating arms to secure/ lower the boat to the ground, and then drive the trailer out, would be my route.