The only hard part is welding on the new spring perches. That cost me $100 for 1/2 hour labor, and I had to take the trailer to the welding shop, couldn't find anyone mobile that would do it on site.
The rest of it is just unbolting things, replacing anything that's really rusted, welding the perches, and bolting it all back together... Easily done in a driveway with simple tools.
$400 sounds reasonable for a double axle... someone can spend a lot of time fighting rusted over-torqued fasteners.
2013 KZ Sportsmen Classic 200, 20 ft TT
2020 RAM 1500, 5.7 4x4, 8 speed