I can't answer that question because I didn't build it, it was built by Texas Tilt (Doug Graf). Doug told me how he did it but, not the cost.
I have helped with smaller trailers (10 years ago) and was told the bids $150 to $600
I have had smaller items galvenized and there was a big difference in the bids (a mast support was $10 to $105) so it pays to shop around.
The best work was done by a big company who "tucked" my job in with a big job for a railroad. The worst job was by a small shop
Some trailer builders that hot dip their own trailers will offer this. I was told a rough rule of thumb was 1.5-2 times the difference in price between a painted and galvenized similiar trailer.
Now you know as much as I do